My 3.5 year old only does his poop in sleep. he doesn't want to do it in the toilet. i'm concerned.

Troublesome problem: This can happen to any kid who has pain with stools as soon as they learn to retain them. By retaining them when nature calls, they dry more, bulk more & eventually pass with pain again. You need to consult with a pediatrician with training in this area or a pediatric GI doc to work on a bowel rehab plan. It may take a year & will require effort, but it can be fixed.