J.A.
I noticed that when that happened to my little girl she was typically teething, so I started putting the Hyland's Teething Gel on her about 15-20 minutes before a bottle and it worked like a charm. Sometimes I would also give her a cold teething ring to chew. That would help to numb the gums a little bit. Have you tried sippy cups? Maybe you can introduce one that is proper for his age, and altenate between bottles and sippy cups. We've done that with my daughter and it has worked as well. Besides, it'll make it easier to wean him off the bottle when he gets older since he won't be so attached to it.
GOOD LUCK!
J.