Yeah, that's definately something you need to bring up with your partner. It might be nothing, it might be something major, you never know until you ask. Try to avoid making him feel cornered, though. I wouldn't recommend busting right out with "You don't want to have sex with me", I would instead try something along the lines of "It feels like we never have sex anymore, and I really miss it." See where it goes from there.

Best of luck to you, I know how disconcerning it can be.