bowel habits

Forgive me if I am repeating myself because I wrote on 2-l7 about my five year old grandson who seems like he can't tell when he has to have a bowel movement until it is too late.  Other times I think he just doesn't want to sit on the toilet.   His mother has resorted to putting him in "pull-ups" so it is easier to change him and not have so many dirty undershorts.   We have tried everything to discourage this problem, but nothing seems to change.  Sometimes he sits on the toilet but has already put some in his underwear.  He goes to preschool and I don't know if he has done this there too or if its just at home and my house.Any suggestions? His father lives in England and the child has never seen his dad except in a picture.