Frustrated by lack of sex

My bf and I have been dating about three years and living together for just over one. He's amazing, and he's been so supportive through some really tough times, but our sex life is basically nonexistent. 
We've only had sex three times in the eight months since I started really tracking it, and maybe 6-8 times since January 2016. My sex drive is through the roof, and his is barely there at all. It's messing with my self esteem (I happened to gain a little weight last year) and making me feel really weird about our relationship.
We talked about this semi recently and he said he's just too tired to have the desire -- he works extremely long shifts 4-5 days a week -- so I know it must be true, but it's so hard not to take it personally.
What should I do? I'm tempted to tell him to make a doctor appointment to get checked out because it seems like his sex drive dropped to zero overnight, but I don't want to make him feel bad for something he can't change. And I really miss the closeness and intimacy of sex.