From what you are telling me, there is a significant weight gain even if your lo is on bm. I assume that your bm is enough if there is a weight gain. Do you want to have your lo exclusively breastfeed from now on? You may do so by trying to increase your milk supply. Perhaps you can do the pumping every 3 hours or just do direct latching. The more you give, the better your supply will be. Also, do continue to monitor his weight against your baby book. If he is within his normal curve without a dramatic decrease in weight then he should be fine :-)
Doesn't sound like any problem here to me. Could baby be teething that's why is is not interested or cannot sustain long interest at the milk bottle. Some babies teeth at 4 months and signs can appear up to a month before that. My boy also mix feeding that time and sometimes I need to make him drink otherwise he won't drink - if he doesn't see the milk, he won't cry for it; if he sees it or drink a bit, he will wana continue drinking lots
It does not sound like anything is wrong. He is gaining weight. I would continue to give bm only and monitor. As long as there are wet Diapers ill say it's okay. Also if a baby is hungry they ain't going to keep quiet. That's for sure. Many moms worry that their milk is insufficient.
As long as he is gaining weight at each doctor visit, you don't have to worry about his intake or his weight percentile. All these are just guage. Secondly, if he is drinking via latching, you dont have to worry.Baby are very good in extracting milk out from mummy
this link could be able to help! http://www.bellybelly.com.au/breastfeeding/helping-your-breastfed-baby-get-more-milk/
do you latch him on or express? if you latch him on, can stimulate more milk for you and there will be enough for him