help for my 3 year old

I have a 3 year old son who has suffered from constipation since he was about 6 months old.That is when he had to go onto formula because I fell pregnant again.Anyway we tried using everything for him.Applesuace, juice's, water giving him lots of fibre and changing his diet.Eventually his ped recommended Miralalx at 1 year old.He was on that for 6 months and then was referred to a G.I. dr .He then put him on a strict diet and a maintenance of Miralax. He was slowly weened off that and was okay for a short while. He then started holding back his poop.It was so traumatic for him and us that we couldn't go anywhere. We then went back to the G.I. dr and he tested his thyroid and celiac which all came our negative.He was then put back on Miralax. I had heard negative things about it so I changed him to Magnesium Sterate.That was working very well until one day he decided to hold back again and really made things bad for himself.He was having severe stomach pains and his personality changed.he became really aggressive and mean towards everyone.He also wanted to sleep all the time to avoid pooping. We called the G.I. dr again and he put him back on the miralax. A whole cap full once a day.He has been on that since last wednesday.He has had 5 bowel movements since then. Does anyone else have any suggestions for me.He does eat a lot of fibre in his diet.We try give him at least 8grms a day.he also has very watered down juice. The Dr talked about a rectal test to measure the pressure in his rectal muscles and also doing a biopsy.Has anyone had this done to their child and do you know what  they are looking for?Any input would be appreciated. I am really concerned as we have been dealing with this for a long time. I must say that he holds back whether he is constipated or not.His poop has been really soft the times that he was holding back and he would tell me that it is not even hurting him. The only time that he doesn't complain is when he is having a cap full of Miralax and then it just pours out of him. I would like him to learn to have healthy normal bowel movements. Thanks again in advance.

[sue_perfluous]

It could be that he's encopretic in which case - poor him and poor you, you've got a long struggle on your hands.  All I can say is he needs to get off the Miralax which has a bad reputation for side-effects (see the Miralax Yahoo Group) - it could be that or the toxins from his constipation affecting his behavior.  If he gets taken into hospital he'll be given a series of enemas - which is probably the best thing to give him at home when he is stopped up.

[protea]

Hi there I dont like the Miralax myself so that is why I switched him to Magnesium.He was doing well on the magnesium for a while and then all of a sudden started holding back. It got so bad that I had to call his GI dr and then I knew that he would put him on miralax again. We have tried diet ,increased liquids and all other things to help him.We have tried reward charts nothing helps.He is now going 2 a day with a 3/4 cap full and we are allowed to lower the dosage to 1/2 cap full in a couple of weeks.He sees the GI dr in March and then hopefully he will have some answers for me.There is a history of constipation in our family.But the thing is he is not constipated when he was holding back it was really soft still so I think there is a mental block somewhere.
That is why his dr will do the motility test on him and also a biopsy to make sure there is no medical reason why he is suffereing like this. I really hope its not encopresis cause I have read a lot about that and it is not fun.Thanks for you input I will update you as he comes along.Thanks again.