As you can already see, you going to get different answers, none of us can give a good answer without knowing/seeing your son, his riding style etc.
I mayseld, would normally be fit to a 54, but I have a 'Medium/long frame, closer to a 56, I like being stretched out.
Your son, if he hasn't done a lot of riding may feel better being more upright & like a smaller frame, again, it tuff to make the call on the info you have given, it's like flipping a coin.