Edit: I've just re-read what I've written... lol- pretty biased stuff right there (even though it's true). in all seriousness, if i was in your position i'd send my sibling to grammar. he has secured a scholarship- he has pretty much guaranteed himself an all-rounded schooling experience (assuming he makes the most of it). just make sure he does his best regardless of the poor efforts of those around him.
totally contradicting myself
I'd go to Ruse.
Admittedly the facilities there are pretty average (there are a few awesome teachers there that go out of their way to help the students, though), but the amount of competition there makes up for it! Ruse isn't really as nerdy as it's stereotyped to be- sure, there are a few super nerds there (in most grades, the kids at the top are amazing at their subjects)... but which good selective/private school doesn't?
The students there are all unbelievably nice; they all support one another, and hence, the atmosphere there is really relaxed (however in senior years, exam time is hectic!). You wouldn't find a nicer school elsewhere, that's for sure.
Oh, and the chess team is pretty awesome! There are quite a few superb chess players there, and the top-end debaters are really good. And you wouldn't believe how musical the school is
(heh, they put on spectacular musicals/ music performances).
However, if you're positive that your brother will thrive in a school with top-notch facilities (as opposed to endless support and friendly competition), then you would probably be better off with grammar.