3 1/2 yr old gags on potty

My friends 3 1/2 year old son, will not potty train. But, not for the typical reasons. He gags and sometimes actually throws up from his disgust of pee and poo. She has tried since 2 1/2 years of age on a potty chair, to the regular toilet with aid, and he gets too grossed out. On top of that difficulty, he is a large boy (size 7-8 in pants width) but his penis is small. So small, it's hard to aim it in the toilet (especially in sitting position). I know he's not alone as a 3 1/2 year old boy not potty trained, but with the other problems there doesn't seem an end in sight. Please send advice. Thanks