Tiktok's the expert, so the extra info she's asked for I'm sure will help in getting you the best advice.
For what it's worth, ds was EXTREMELY slow to gain weight, although he never actually lost weight (apart from the intital and very large dip at birth): I think he was about 6 or 8 weeks old before he regained birth weight and he was 6 months old (I think) before he doubled his birth weight.
Reiterating tiktoks' point: we eventually worked out that he was following his OWN growth curve. He was always happy, healthy and alert, with plenty of wet and dirty nappies. At the end of the day, that is the most important thing.
With the help of the breast feeding support group and the midwives who ran it at my maternity hospital, I had the confidence to continue to b/f for the full 12 months.
What helped me was being encouraged to give him EBM - for a while, for every second feed. That way we could see that he was having plenty, and the fact that he was still only gaining weight slowly just meant that he must have a fast metabolism or whatever! We did have some problems with the latch, which we worked on, and with the little tike only pretending to swallow, but the supply of EBM gave me the confidence that he was definitely getting enough.
In the end it was the consultant paediatrician who told me to stop "having the faff with all the expressing" and go back to plain b/f, as I had a healthy baby who was just following his own chart. (I actually kept going with the expressing as I was going back to work when ds was 4 months old and it was useful for us both to have the habit and to have the stocks in the freezer).
If they've taken blood tests to rule out anything more serious and if your baby is happy, healthy and alert, plenty of wet and dirty nappies, not dehydrated (skin is plump and soft and doesn't wrinkle when you pinch it), then you should try to stop worrying. Ideally you should consider STOPPING getting him weighed and use your own observation as to his state of health instead. I never actually succeeded at that (until I went back to work and it got increasingly difficult to take him along to be weighed): I claim I wasn't WORRIED as such, but that I had a morbid curiousity about his weight. But at the end of the day, what does knowing his weight achieve??!!
Ds is now nearly 2 and half and I don't have a clue how much he weighs. But he's healthy!