4yr old son is aggressive

Im 24, single mom of two and live with my parents.
My son is 4 and started school this year in pre-k. I was hoping that interacting with others would calm him down and open him up, but i was wrong.

His history. He was born at 41 weeks, had no problems except for the webing under his tongue need to be cut so he could feed properly. He was colic bad up until 4 months, after that he developed cronic (chronic) constipation which went undiegnosed till age 2 where he was tested, xrayed, seen a specialist and had more things done to him a grown man would run from. He is an extreamly picky eater which makes it very hard to make him eat regular food. He is literally 28 lbs at 4 yrs old and was 20 lbs for 2 years. His sister is 5 (10 1/2 months apart) and they have the usual fights like all siblings do.

Okay more for mental issues: his father and me are not together due to fights. At 2 years old I remember his father screaming at him one day because he came home very stressed and i was busy with cooking so i couldnt really tend to the kids(they were fussy and crying).( wonderful military for you) soon after i left my husband because of abuse.
Once my kids and I moved I noticed he was having nightmares and crying in his sleep, it took awhile for him to get over those. He started to act out at 3 yrs old. Screaming, crying, throwing himself on the ground, hitting his head on things, hitting himself and just being dificult.

He started being agressive to his sister who is terrified of him when he gets angry. He will throw toys at her, hit, kick and even bite her. At school I get the same talk. He hits other kids and hits his teacher, Screams, throws his food and will not calm down. He has these fits several times in a 5 hour period.
I've tried talks with him, being calm, time out, taking away toys and it just doesnt work. He is so short tempered, anything will set him off, even positive praise when he is good. very quiet, shy, loving.

I think all this might be tied to the over whelming pain he delt with due to his dad doing what he did and us fighting when he was younger, him being colic, and then the cronic (chronic) constipation( he wont go #2 for a week to 2 wks) all in a very short time period. Can agression be passed down geneticly? His father is very agressive and dificult as well, ive noticed even his family has "issues" like that.

Im truely lost, ive asked the doctor about what can be done and she is clueless. Ive asked the school and they are pointless and push us aside( love the public school for that).

I find myself crying almost every morning because of this, im scared im gonna have the next kid who is thrown in jail for his temper, or be thrown out of school.  Im sorry if this is all jumbled, im still upset over hearing his teachers tell me he was hitting others again. i know there was something else i was wanting to say but with stress im a bit forgetful right now.

[AHP84]

I think your first step needs to be finding a different pediatrician. If his current doctor is "clueless," then she needs to be making referrals to child psychologists or neurologists, etc. if she's had you mention these issues a few times. If your pediatrician is shrugging you off and saying "I don't know, maybe it's just a phase," then there is certainly no harm in seeking second, or even third opinions.
As for the past issues--I'm wondering, does he see a pediatric gastroenterologist? Or have you found a solution to the chronic constipation now? If not, and he's still not going at least 5 out of the 7 days per week, then I can only guess his gastrointestinal issues will get worse with age. I suffered chronic constipation as a child, from the day I was born, and I have severe IBS today and had a pre-cancerous colon polyp removed at age 22 in a colonoscopy. A lot of advances have been made in pediatric gastroenterology--I'd highly recommend having your son be seen on a regular schedule with one since he's had not only the chronic constipation his whole life, but also the colic. The colic makes me wonder if he maybe had reflux or GERD--which he may still have, and may even be an explaination as to why he's a picky eater (certain foods upset his stomach but he doesn't know how to tell you that because he can't recognize it himself). My son had GERD at age 3 and I found out from a pediatric gastro when I took him in for a colonoscopy (for a benign issue of blood in his stool), at which time they also did an endoscopy (to check his stomach). They found out he was what they called a "silent sufferer" of GERD because he'd probably had it most of his life, so he was used to the pain, but not yet capable of expressing with words that he had an upset stomach or was burping up gastric juices. His GERD had been unnoticed and untreated for I don't know how long, and the result was that he had the beginning stages of an ulcer just near his esophogus. He was on Prevacid for a couple of months and now he's fine (he's 7 now).
The anger and aggression--yes, I do believe that can be hereditary, BUT I think it is also something that can be triggered moreso than "pre-programmed." I think what he sees and experiences in his environment will play more of a role in how he acts out than genetics alone. Does he watch violent or fast-paced cartoons often? Does he still see his father through visitation? Does his father still act aggressive and verbally/emotionally abusive towards him if he does still have contact?

I think all of these issues do play a role in his behavior. The key is getting to the bottom of it--and if the main people who should be helping you (the pediatrician, the school) are not, then find someone who will. You are his best advocate, and believe me, I know how hard it is to have "professionals" take you seriously when you just KNOW something isn't right.
Best of luck to you.

[Sandman2]

   Very good ideas by AHP and worth considering all of them.  I think I would also check out if he has any throat obstructions causing a lack of sleep.  This would be something like swollen adenoids (adenoids).  Kids who don't get enough sleep have many of the characteristics you describe.
   I do think part of the problem could be his diet.  If he is a finicky eater, he may not be getting enough energy from his food and that will cause problems at school and home.  The constipation is certainly a problem that must be worked on.  
    Also, kids do need to be taught how to handle their anger.  There are a couple sets of books aimed at this age group.  One is, " How to Take the Grrrr Out of Anger" (Laugh And Learn). That and several more are found here - http://www.amazon.com/Take-Grrrr-Anger-Laugh-Learn/dp/1575421178/ref=pd_sim_b_7
          Another good set is the," Don't Rant and Rave on Wednesdays!" The Children's Anger-Control Book. That and others in the set are found here - http://www.amazon.com/Dont-Rant-Rave-Wednesdays-Anger-Control/dp/0933849540/ref=pd_sim_b_1
          Your comments about his diet also made me wonder if he could have something like SIDS or sensory integration disorder.  You can check that out here - http://www.sensory-processing-disorder.com/sensory-integration-dysfunction-symptoms4.html
   Hope this helps.