Its tough, but if he looks healthy then he is probably ok. If he us unsettled during the day then you could try another feed. I have been down this path twice now and it's not easy
. Ds only put in 200g over his birth weight at 8 weeks, it turns out he has acid reflux, by giving him Zantac and offering one bottle if ebm and 120 of formula he has managed 260 in a week!

I hope you can find a solution.