The little bubbies go through growth spurts at 6 and 12 weeks, so it looks like your little (BIG) man is doing what he's supposed to.
I'm surprised the health nurses haven't explained to you the concept of demand feeding during spurts. Silly women!!
They will just feed and feed and feed for a few days, then they sleep andsleep and sleep for a few more. Its nice when they go into the sleeping phase