I had to laugh at this. DS LOVES building Lego sets. So much so that he independently built an enormous train set for DH's (adult) friend because the friend wanted the finished model but didn't want to put it together.
Our library has tons of these type of Lego books:
Which DS loves poring over. Could be a non-stress way for your DS to engage in his hobby.
Also, keep in mind that it sounds like this is a real bonding, social thing with his friends. For that reason, I would probably find ways to honor his interest (which it seems like you are doing) without going against your family's values. Could it be that all his talk and exchanging of catalogs is really more a social thing than him actually wanting more, more, more? Also, maybe Lego playdates are in order!