Gagging on his cum

I cannot give my boyfriend a blowjob without gagging. I have gotten better at getting it deeper down my throat without gagging, but if he cums in my mouth I have to quickly run to the bathroom and spit it out while dry gagging and sometimes almost throwing up. I feel so bad because I know it makes him feel insecure, he always asks if it tastes bad and I try to tell him that it isn't the taste or anything he does. Does anyone else have this problem?