toilet training 5 year old

my son is 5 years old and was diagnosed with asburger syndrome at 16 months old.  i was just wondering if anyone has any ideas on how to help in the bowel movement department.  he refuses to go poop in the toilet, it doesnt matter what ive tried he will wait till he has his diaper on or drop his pants and go outside or in a bedroom, some place hiden. ive tried rewards, timeouts, talking to him telling him what a big boy he is and nothing seems to work.  any sugestions will be helpfull thank you.  also is autism heretitery. my maternal sister has six  kids one with very bad autism, two  with  asburger  , i also have a maternal cousin who has two kids one major autistic one minor.  is there something in my mothers side of the family that is a contibuting factor any help will be greatly appriciated.