I don't think that letting him do it in his nappy would be a big step backwards. If he's scared and he's hanging onto it he can get constipated which could make it quite painful to poo and make him even more freaked out by the TT gig. Some kids poo in nappies for a while after they do wee's in the potty. It's not a race and he'll get there in the end.
If you're worried about constipation a bit of fruit juice can help move things along. If Yasin hasn't pooped in a while I usually juice up some carrots. It works a treat.