Breastfed weight gain

Sorry I deleted the last trying to edit it

So Im trying to get my little one to gain more weight, he’s 3 1/2 months and exclusively breastfed. For the first 8 weeks he was on the 95th percentile and since then he has dropped to the 50th percentile. He only ever feeds from one breast and never wants the other so I don’t think its a supply issue. I have tried to get him to nurse more often but he will refuse it and it gets him upset. I have read up on separating for and hind milk but not sure how much to dump. I can get about 4-5 oz when i pump and he will drink about 4 from a bottle so do i dump the 1st oz? Also is there anything i can eat to make my milk fattier or any other advice will be very much appreciated. As a note I had the mirena IUD at 8 weeks and this was never an issue with my first baby