Hi hun,
Exact same thing happened with my little boy. He was 7lb 7 born, lost nearly a pound and then took ages to put weight on. We were referred to a diet specialist etc etc and they kept threatening to make him have formula but as long as he was alert, responsive and healthy in all other ways I refused. I did start him on solids a little earlier than normal (I think it was 18 weeks) but apart from that he was fully breast fed. He is now 2 1/2 and is below the bottom line still but has grown steadily under the bottom line since he was about 2 months old so he's obviously just a thin little boy!
Stick at it if you can hunny I'm sure your both doing great xxxx