CHANGE SCHOOLS! Joking. This is actually a great opportunity for him to start to accept things as they are. It is SO normal for little kids to covet things that are bigger and cooler, and sometimes things that aren't bigger and cooler-the grass is always greener! His heart break isn't genuine at this age about his siblings etc. It's nice he's talking to you.
We were super poor and grew up way rural in Colorado in the 70's. Most of the other kids lived in trailers and had biker parents. But ONE girl had this MANSION (probably if I saw it as an adult it was a normal 2 story :) with a (gasp) PIANO and LOTS OF BARBIES, not just one with hacked off hair and ink stained face, like me. I would go ON and ON about how grandiose and amazing her house was blah blah I wish we had that blah. And then there were our cousins with a HUGE POOL in Minnesota (smallest suburban house and yard ever but we lived in the woods so it seemed FANCY).
I think what was really cool about my parents, was they never got annoyed with the comparisons. We were brought up in church with the selfless giving thing, my mom was always volunteering and donating and helping others even though we didn't have much, but they never railed against materialism to us.
To this day, I teach my kids there is nothing wrong with having a lot just like there is nothing wrong with having a little. Being thankful for what you have counts. When they tell me about everyone else's nice stuff in other towns or on TV, I just say, "Wow, that IS REALLY NICE, if you work really hard one day, and you want that, you could HAVE IT!" So far, they're not jealous of it, they love it, and don't feel threatened.
I would just be supportive of him and agree with him and keep it light. Remind him he can have all that one day if he wants, but it's not nice to say he's unhappy with his own family (not harshly, but you get it)because you guys are fortunate and blessed. If he gets nasty or starts to really dwell on other people being richer, you may want to vary his routine a bit and get some poorer friends as well, but this outburst was OK (maybe, the sadness and crying could be an alarm though -good thing you're on it) but the basic competing is normal. As long as you are happy and thankful for what you guys have, modeling that, and not letting him turn mean.
There's a down side to everything, and for all the perks of his great expensive school, he'll have these issues to conquer. We live in a really down to earth setting where our humble house is the norm, so we won't have those issues so much, but our local public school sucks and is the only option and I'd switch problems with you in a heart beat! :)