I'd also find out if he's as eager to go to camp as you are. If he is
uncertain, and knows that no potty training=no camp, well, you've given him
an out without admitting why. And that could explain the balking.

And, FWIW, there are programs for 3 yr olds which don't require toilet
training, they're just harder to find. My 2 1/2 yr old is going into a older
2s/3s program this summer which is largely 2 yr olds who are a bit too
verbal and too "old" to be happy with young 2s and 3s who aren't potty
trained yet. They do mostly the same stuff as the 3 yr old class, but have a
lower ratio and the expectation is that the kids will be working on toilet
training.