Potty training

We have been potty training Connor for a few weeks now and he has gotten it down faster then Ethan or Zachary has, well as far as Pee is concerned. The poop, he hasn't quite figured it out, that he is suppose to do the poop in the potty too, so he has had a few accidents. When we go places and when he goes to bed, he wears a pull up and of course to him, its a diaper, so he does everything in his pull up. He doesn't use a potty seat, he uses the toilet like everyone else does, which is a good thing. Zachary is 6 and he is afraid of the toilet, so he still uses his potty seat and we keep telling him that he is going into the first grade and that there are no small toilets like he had in Kindergarten. Does anyone have any suggestions of what we need to do to help Zachary get over his fear? He starts first grade Aug. 14. Plus, Zachary is afraid of so many things and we don't know what to do about those things. Some of you don't know this, but Zachary has a mild form of Autism and he also has Muscular Dystrophy, so he isn't your typical 6 year old.