does everyone else in the circle have to match his raise?

Yes. Betting concludes once everyone's put in the same amount (or folded, or gone all in).

does he have to put in 15 MORE or just another 5 to match the raise?

Just another 5.

Of course, this is just my understanding of the rules. I'm sure Rob will be along in a moment to explain that the way we play Texas Hold-em at "Poker Night" in Cambridge is the simplified rules, and that I'm wrong.