2.5 year old with severe, chronic CONSTIPATION...please help!!

My son is 2.5 and has suffered with constipation for over a year. He has been on miralax for almost that whole time. I have tried so hard to stop his need for a laxative, dietary changes, more fluids etc. but then it gets so bad I have to resort to a baby enema and then I start the miralax again to avoid another very traumatizing enema/suppository experience. He will not go for three days and than strain for an entire day without any bm or just with a small ball of hard stool. I I don't give him something to help him pass he will be fitful and awake most of the night. I don't want him taking laxatives for another year or more!!!!

Some background... he has a twin brother and has always been much smaller than him. At birth he was 1kg lighter and the gap has just increased. Though small he is very strong and
rarely sick (other than constipation). At 2.5 he is only 11 kg. He gets very angry when he doesn't gets what he wants, and can sometimes be scared of "monsters" when we're going to sleep. Other than that he is a very easy and respectful child.

Last year I had another baby when he was 15 months and 1 month later the constipation began. I feel like there may be some emotional connection between the new baby and the constipation. Both twins have suffered from constipation, however the bigger twin's issues have (mostly) been worked out with diet. Any suggestions for treatment or success stories would be beyond appreciated!! :) Thank you!