I don't don't want to sound harsh, but OP you are not being a good partner here. Even from the little you said, it's pretty clear that your BF is not into getting random nudes from you. No person should feel pressured to like this kind of content, plenty of people of both genders are not at all into it. Surely after knowing him after four years you should realize that it's not his thing.

But not only do you not stop from sending nudes, you even pressure him to respond in a way that you would like and guilt him when he doesn't. You even call it "progress" when he half-heartedly said he would play along. This is not how a loving partner who respects her boyfriend's comfort and desires should act. You are being selfish by pushing a sexual element of the relationship that your BF doesn't like and are then emotionally manipulating him into acquiescing.