Some thoughts -- is there a computer lab or library or some place he could go to during his "math" time to do independent work? Maybe not math work, though, if you're looking for something challenging. Maybe some sort of applied math type thing -- programming or engineering type games? And then do math at home, perhaps with a tutor? Does your ds write proofs? That might type of thing might be a nice challenge?
There are such places, but I don't think there's anywhere that's routinely supervised, and I don't think they (or I!) would be that keen on a 6yo being regularly unsupervised. Maybe an idea for in a few years' time. It's true he could just do something else in maths time, but it seems a bit of an admission of defeat! He does enjoy programming and writing proofs (natural deduction proof trees - is that what you meant? I'm surprised it would occur to anyone else as a possibility, but maybe I shouldn't be :-) but if he's going to do genuinely challenging things in either mode, we're back at the problem of his lacking the maturity to deal with the challenges without any input. He could of course just read (about maths or about something else), and there would be worse ways of spending the time...
ETA: maths at home is fine, yes, and no tutor needed, as DH and I are mathematical and he doesn't as yet have any problem learning from us.
