Thanks for your reply Jennifer.

He has always been this way with his feeds. Some feeds he's lazier than others and some feeds he's really good. He's definitely getting enough (he gained 540g in 8 days one time!), and has plenty of wet and dirty nappies.
I'm using the nipple shield because I have very flat nipples. I have heard that it can mess with your supply, but that is not a problem - I have heaps!!!
I'm wondering if he is getting quicker feeding, cause in those first five minutes he guzzles it down, and sometimes sounds like he's drowning in the stuff!