We've been together for about 4.5 years and we're both 22, so we shouldn't theoretically be having any problems. I lost my virginity to him when I was 17 and a senior in high school. I went off to college and we stayed together with some ambiguity about the definition of our relationship for about the first year. When we would see each other, mostly at his parents' house or mine (my mother moved out and the house was empty most of college) on the weekends or during breaks, we would have a more than decent amount of sex. He was all over me, as a teenage boy generally is, and I was into it. We wouldn't only have sex, but it was absolutely never a problem. Our relationship was great.
He moved into my mom's house and rented it from her about a year ago and now I've graduated and we're living together in my mom's house and have been for the past five months or so. Our sex life seemed to instantly drop off after I moved in. He still wants oral sex all the time and still masturbates daily, but if we ever have sex, it generally falls flat because he's forcing it. There have been a couple times in the last couple months where we've actually had sex. I'll admit I'm not the greatest at it and kind of awkward, but that's never stopped him before; he'd always say I made up for his doing all the work during regular sex with oral sex, which was fine. But like I said, if we have sex now, it's normally just sad. He'll get me all excited and worked up and then not want to have sex but still "would not mind" oral, and I obviously just feel shut down and really don't want to, and generally don't. Sometimes we'll have sex and, even when I'm facing away from him, I can tell he's absolutely not into it and I stop it because it's really disheartening to see someone who was so into sex before just not care.
As I said, we're both 22, I'm skinny but not weirdly skinny, and he's very fit. He's always been into exercise and cares about his fitness and diet. Life has seemed to lose a bit of its luster for him recently (in the past year or so), but it never seemed like it was a big problem and he carried on and I tried to keep his head up. He also says that it's not my fault he doesn't want to have sex and that it's all him. This doesn't really make me feel much better; I just feel helpless. He does smoke marijuana pretty much daily, but he has since I've met him and that has never really affected our sex life positively or negatively; it just is.
I'm not a terribly sexual person, but I do really enjoy having sex with him. It used to be a lot of fun. I'm not going to go find it elsewhere and it's not something I really need in a relationship to be content; it just worries me because it was such a staple for so long and now, not so much. I'm more concerned about him than I really am about our relationship, but I'd like to know what's causing this.