Bedroom Trouble?

My guy and I have been together for 5 years. I'm 19, he's about to be 23. We have a had a pretty good sex life for most of our relationship, and we are pretty perfect for each other. We're comfortable. We typically screw like rabbits whenever we can. Once a day, at least, when we're around each other.

But lately, for the past month or two, he hasn't really wanted to very often. Like, a max of 6 times in the month of January. And while I can take no for an answer and won't force him to do anything, I feel left out. Idk. He doesn't even really masterbate a lot lately (he usually has a healthy relationship with himself in that regard). (We're also pretty open about these things with each other.)

I know it may just be an overreaction. But tonight I wanted to, and when he said he wasn't up to it, I felt really sad. I told him it was okay because I don't want him to feel bad, but I just got really disappointed.

I masterbate every now and then when I am not around him, but even though I can reach climax when alone, it isn't as fulfilling as sex with him. So I don't know what to do. Could there be something wrong with his ability to become aroused? Or could it be that an external issue in his life is causing this?

I appreciate any help that you may be able to bring. He doesn't know why he's been like this and neither do I.