He's uncomfortable and doesn't want to tell you. I'm in a similar position myself; my girlfriend always initiates any sexting because I feel super weird doing it. The first few times, I never really reciprocated and I never told her. But whenever we'd see each other in person, I'd be all over her. This quarantine made me realize that this is the closest I'm getting to sex for the next few weeks (months?), so I tucked away all of my insecurities to help make her happy.

Is your boyfriend body-conscious? I have a little belly fat and it makes me nervous to be shirtless around my girlfriend even though she always compliments me. That insecurity is exacerbated when she wants me to send her pictures. There's gotta be something going on up there for him. Hope it works out for you though.