I'm with huggles on this one. A gain is a gain. As long as he is fine and sleeping well (ish for a baby) can go stretches. I don't know how long you go between feeds for them but perhaps Emma can go 4 hours but Japie only 3.
Today our second little girl is 4 months old, she is breastfed, if you give her a bottle of expressed milk she will drink 60 - 80ml. That is why I try never to express because then I don't know how much she drinks and can't compare it with another baby her exact age. I have also only had her weighed once at 2 weeks to make sure she was gaining from breastmilk, again, so I can't compare. A friend of mine's baby lost 150g in 4 weeks, she is stressed beyond comprehension, which I wouldn't be, not for what I know of her baby anyway.
Trust your gut, not the numbers on the scale (as a society we have a weird fascination with numbers on a scale!) I know this might be a bit harder for you given your history, but if he seems fine, looks fine, acts fine, then I guarantee you he's fine!