Saw the doctor today, again he gained a lot of weight but she is not too worried about it at the moment. She says he needs to make up for the time when he was losing quite a lot. She prescribed domperidone, she thinks it will help to kick-start my supply because otherwise I'll never get to it since during the day he doesn't nurse a lot, so my pumping sessions help, but are not as effective since they don't come after a really good nursing session. She said within a week I'll be able to see a difference, and then hopefully he'll be more interested in BF during the day, so the supply should come naturally after that.
Hope it works!