My ex wants to talk to me after nine months?

Okay, so me and my ex are good mates, we mess around a lot and I've gotten to know him through that last few months, mainly because I got to k ow him through being a couple, so I only knew that aspect of him. His girlfriend is one Of my mates, and she's really frigid, he's always asking me for advice, and I try to help an that, but I dunno how! They've been going out about 8 months and have kissed twice, he's getting really annoyed with it, because they never talk really. Anyways last lesson today he told me he needs to tell me a secret and he'll tell me on tuesday(royal wedding and bank holiday Monday we're off) I asked him what about and I said is it your girlfriend, and he just stared at me... I do still like him, not that I'd ever tell him, but what could this secret be, he hasn't told any of our friends (cuz we all hang round in a group), not even his best mate, whom he tells everything to. Please help.