Honestly, it's perfectly normal for babies to "apparently regress" once they've mastered a skill and get bored with it. He also could be having a VERY short-term, self-limiting reaction (meaning it's nothing to worry about and will clear up on its own)- if he has some fluid in his ears, it could be impacting his hearing temporarily. If he doesn't hear his name when it's called, how can he respond to it? How is he reacting to other sounds? Is he babbling as much as usual?
Honestly, I think your baby is completely fine. However, if you're THIS worried about him, then it shows your distrust of vaccines. I suggest you give serious thought to this before your son is "due" for the next round of vaccines- research the vaccines themselves, as well as the diseases they're designed to protect against. Talk to your husband about the research. I don't think you should let your son have any more vaccines until and unless you're 100% comfortable with the idea.
You could stop all vaccines completely. You could give vaccines on a delayed schedule and/or only give some of them. You could give him only one at a time and have the whole vaccine series take longer to complete. You could hold off on all vaccines until he's 1 (or 2, or school-aged) before "catching him up" with the recomendations. You have options, other than "vaccinating according to the AAP schedule."