All good stuff written already!

If it were me I'd probably stop asking altogether (but my kids LOVE doing the opposite I want!), and let him tell me every time.

Apart from that, the camping trip might be a really good thing. He may just step up to the plate (dunny) so he can be like the other kids. My 2 will eat lots of things they won't at home because the other kids do. Give him an "out", like I know you will do it when you are ready - so them he can decide when that is. It also gives him something to say if the other kids ask about it "I'm not ready yet" for eg.

GL xoxoxox