This is a tricky one, because children are very materialistic and there's nothing much you can do.  Even if you had the money, it wouldn't be a healthy atmosphere for you to start competing over who can buy the most for him.  Also it did cross my mind that maybe they are treating him to lots of "stuff" but maybe not giving him attention and this is his way of getting attention from his mum?

I would talk to mum and ask her to stick to the set arrangements and try and ignore it.  If you don't feed the monster, it stops growing.  He'll learn that his boasting doesn't impress you and it will stop eventually.  Alternatively, go along with what he wants.  He may be pleased in the short term, but he'll soon start missing the 3 of you and want to come and see you.  He'll get just as bored with them as he does with you - he's a kid, that's part of his job.

Clare x