Hi luca,
The first place to start with your mattress research is post #1 here which includes the basic information, steps, and guidelines (and many of the most useful forum posts) that can help you (or your son) make the best choices.
I’m happy to help with how to choose but there are too many variables and individual preferences for anyone to make a specific recommendation about what mattress to choose out of the many thousands of mattresses that are available (see mattress firmness/comfort levels in post #2 here).
The better options I’m aware of in the New Haven, CT area are listed in post #2 here.
Phoenix