Keep in mind two things:
1) The Poly or BC or any deluxe will be a lot more money than a moderate resort.
2) It might be a little disappointing going from the deluxe resort to the moderate. The deluxes have MUCH larger rooms, nicer amenities, perhaps a nicer pool (although POR has a great pool IIRC). I realize this may mean your DS wakes up in a moderate resort on his birthday. However, if he liked CBR he'll probably love the other moderates. Then you can expose him to the deluxe resort afterwards--sort of like building up the excitement with a stay at a deluxe being the last and best resort experience. Plus, you then don't have to worry too much about the possibility of him being disappointed with CSR or POR after staying at BC. (Don't get me wrong, though. I think the moderates are great too, and you'll love all the resorts).
Just something to consider.