I had a DS like this - 20m he decided he was going to TT himself. Then he decided nappies were easier, so for the next 18m he was in nappies. (Or could be we were visiting my mother and she was obsessed with "no accidents" and having DS TT'd, despite me not being worried.) Even holding it in for hours and hours until he was asking for a nappy rather than use the potty he had been sitting on prior to getting up to ask for a nappy. I also tried putting nappies on the potty so he still had the nappy feel.

He then decided he wanted to use the loo after his friend visited and used our loo, very few problems after that.

So I'd let it be the child's decision every time. Then again, we used cloth nappies so I didn't have a cost issue.