Bottles!

my son started out using the avent natural bottle. The nipples are close to breastfeeding since I was unable to. He will be 6 months old next week and has started leaking so much formula out the sides of his mouth while using these bottles. I tried switching to the dr Browns bottle with the regular nipples. He doesn't leak at all. He seemed fine with the bottle at first and now he pushes it out of his mouth and cries. What is going on?! And how can I fix it?!