He wants me to swallow

Hey girls, at the beginning of the relationship with my boyfriend, I used to swallow. For some reason, it was easier. Now I gag so often and just the thought of swallowing makes me wanna puke. My boyfriend naturally noticed and kind of commented on it couple of times “why don’t I swallow?” I tried to explain that the blowjobs got much longer and intensive, therefore I gag and don’t want to swallow too much. It’s obvious that he doesn’t get it tho. Any tips please? It’s kind of irritating me at this point 😞