my 3 yr old son will NOT potty train

I have 3 children, 3 yr old son, 5 yr old son and 8 yr old daughter. i trained my older 2 when they were i think about 18 months old.... was a breeze no trouble at all and they were completly trained by the age of 2. Now my other son who is 3 will have no part in this training. its like he is scared when he sits on the potty or toilet, he just sits there screaming and crying saying " wont work mommy" as in the pee or poop wont come out. also he is shaking like a scared shake im concerned about this... could it be because he is afraid of hights?? or maybe that he will fall in? i have brought his brother in the bathroom and got him to show my youngest how to pee and still nothing. ive tried the candy and toy reward.. tried putting cheerieos in the toilet to see if he can pee on it...nothing!! but put him in a pull-up/diaper/ undies or pants and then its no problem at all. he will relieve himself annnd SIT in it untill his bum is red and pretty much bleeding. Im fustrated and i dont know what else to do. He is going to start school next year JK and they dont alow untrained kids in this school...Ive never taken him to the DR's about this problem, should I?

[AnnieBrooke]

It doesn't matter when his older siblings trained, he is not ready.  If he is screaming and crying in fear, stop forcing him.  I'm sorry if junior kindergarten a year from now, is looming so large that you are scaring him like this.  You might end up with an untrained 5-year-old if you don't give it all a rest.  The poor kid, saying "Won't work, Mommy," and having you not pay any attention -- your job as Mom is to be sympathetic first, and a disciplinarian only after that.  Please just put him into pullups or diapers for a while and tell him it's OK, and that you know it will happen when it is time, and then really, give him a break.  

[janene140]

listen here annie! for one im not forcing him to do anything hence the fact he still isnt trained!! And i dont stand there and yell at him instead of telling him its ok lol your retarted... quite obvious you read my post and took it the wrong way! like you didnt have to be a b!tch about it ... im just fustrated is all and i posted on here because i thought i could get some GOOD advice. eg; ways to get him to go.... and or why the huge fear. seriously, next time dont bother responding if thats the way your going to be, your are no help to me ... just irratating!