HELP!! I need some advice!

Hi everyone! So I’ve been in an LDR for about three years now and there’s an issue I’ve been having with my boyfriend recently. We live in two different states and we’re only able to see each other during the summer or winter break so almost all of our sex life is through our phones. Recently, my boyfriend has told me that he can’t masturbate without me there. For context, my bf had a rough past with his exes, which led to him having a lot of issues when it comes to anything sex related. I’ve been very supportive and make sure him and I consent. However, whenever I just don’t feel like getting sexy or horny, he gets really bummed out and sad. We talked about it before where I told him that I’m not rejecting him in anyway, I simply just don’t feel up for doing anything dirty at that time. He tells me that he feels disgusted for feeling horny so when I say no at that moment, he shuts down. Even now when I try to be dirty with him, he will tell me that’s it’s okay and he doesn’t want to bother me. I’ve felt upset at myself for not being able to help him, but I do know that it’s okay for me to say no. Sooooooo my question is, how can I deal with this? Is there a way to reassure him in the moment to let him know that what he is doing is okay? Am I able to help him get through his past?Any help would be greatly appreciated❤️