FJ, this isn't as uncommon as you'd think, some friends of mine had exactly the same thing with their two-year-old. She fell in the toilet and got scared, and insisted on nappies. She'd come and insist her Mum put one on her, then go into a quite room and do a poo, then come out to be changed back into her knickers.
They eventually coaxed her back onto the potty, I think they ended up using bribery! I don't know if it's worth making a major drama over it, to me it seems like one of those things that you say "in ten years will it make a difference if I let him use nappies for a few weeks" and go with the flow, even if it's an inconvenience now.

Only you know your son's moods and triggers, so go with your gut feeling on how to deal with it!

Kerrie