Will you have a wheelchair/stroller for your son at the resort? That might be your best bet, as the first two floors of BC were very busy when we were there, but our 3rd floor room overlooking the greenery facing EPCOT was very quiet.
Two things to note though - you can never be 100% guaranteed to be in a quiet room, as your neighbors could be the Rowdy Clan from Make-a-Noise. Lots of people let their doors slam against the safety latch, which is very, very loud in the middle of the night, IMO. Also, BC is very close to EPCOT, and Illuminations is pretty loud, especially if the wind is blowing from EPCOT towards the BC.
maybe the YC side would be better?