Peta, i went through similar, so i can truly sympathise how worrying it is. We had good initial gains, but then a few weeks of NO gains, then 30 grams etc. He was always in the 10th percentile for weight but my Dr said that so long as he didn't drop more than 2 percentiles between fortnightly weigh ins, he was okay. Some babies just grow slower, are smaller (we are both small people my husband and I). Once his severe reflux began to ease (around 6 months) he made solid gains and now is getting chubby. I did supplement with formula from about 3-6 months off and on, so you could do this if you're really concerned, but i would probably wait and see first as it could be detrimental to your supply, and it sounds like he's going great otherwise.

Also, things like having JUST fed him before a weigh in could influence the scales, and then having NOT fed him the next time, it might not be so accurate as we can't always control these things.