I've been attempting to BF my son (been bottle feeding him EBM for over a month now) and one major problem I have is that his upper lip is always tucked in when he sucks. He opens wide and the bottom lip is fine, but his upper lip is always "tucked in". How can I "correct" this? He does the same thing when he sucks on the bottle nipples. It's not an issue when he uses the small nipples (like Dr. Browns), but is when he uses the larger ones like the Playtex Drop-Ins. Any words of advice?