loose BM's and potty training

my step son just truned 4 and we have been trying to potty train him for a year and a half. the problem is he has loose bm's and even has them in his sleep. most of the time he doesnt even stop what he is doing and has a movement. we took him to a doctor today and we were told to keep him away from milk and sugar. well he doesnt have much sugar but does drink alot of milk. he asks for milk when he wants a drink. also it seems to get worse when he doesnt go to his mothers for a few weeks. i am looking for more advice on what to do about his movements. Also when i do try to put him on the potty he screams and crys.

[RockRose]

Has he had a history of constipation?  

It sounds to me like he may have encopresis,  a stretching of the lower bowel from constipation.  Impacted stool allows only very loose stool to pass by - the child appears to have diahrrhea,  or loose stool.