leaking out of mouth and gas prevention

I'm combo feeding (breastfeeding, pumping, formula feeding).

My LO is an intense sucker and he can also get very gassy easily.

We did some research and started out with Playtex DropIns so we could push the air out of the bags, but the bags were too annoying for us to handle.

We then switched to Playtex Ventaire because we heard that it is one of the best for gas prevention and it didn't have a million pieces like Dr. Brown's did. We also heard Playtex slow flow were closest to the natural breast nipple.

Problem is that when he feeds, he leaks out so much from the corners of his mouth and it soaks his bib and clothes. I can't tell the reason why. I know sometimes the nipple flops around a bit or gets pushed in toward the base, but it doesn't happen often.

We had a sample Mam bottle which seems to be a cleaner feed, but that nipple makes me cringe because it is so flat and narrow (which is what my nipple looks like when he's got a bad latch! ouch!). I also am concerned about the air control on the Mam because he has gas issues.

Any thoughts, tips, suggestions?