I'm going to echo what's already been said...there is zero "obligation" to have sex with him. Sex is something you should make as a choice on your terms, not something you feel like you "have to" do. Don't make him feel as though you have any obligation. You do not.

That being said, as far as broaching the subject, if you do WANT to be intimate with him again when you see him (note: want to, not feel as though you should), Hamster's suggestion on how to broach the subject is superb. I was going to propose a more cut and dry phrasing, but then again, it's not my field of expertise there.

Take care, and be safe.

Hugs,
Harley