My DS was the same - would be fine in pants, then need a nappy to relieve himself. Wouldn't use the loo at all.

Until... DS's bestie visited and used our loo. Overnight TTing, few accidents, dry at night too. Fantastic! He was about 3.5 too, but almost 4 before he was confident enough for pooing on the loo (he did have to tidy himself up after accidents).