Another thought...
My lo (9 months now) still wants to suck/chew when he is uncomfortable with a burp or other gas. That is often how I know he is gassy. Also, he won't nurse or take a bottle in this state (it also happens when he needs to have a bm, which can take a while). I know this would go against what everyone else has said but it could also be that the hole in the teat is too big and that your lo is swallowing a lot of air. I just switched my lo's teats to medium flow from fast because he was becoming very gassy and it seems to have helped. You may want to think of that next if the larger hole doens't help.
Becky