Breast/bottle fed baby. How much should he be taking?

So my pediatrician wants me to start feeding my 3m old 5-6 oz a time and spacing out his feedings more. When he was really little I fed completely on demand and tried to pump and breast feed as much as I could but I never really produced much milk. We found out at 5 weeks he had a tongue tie and we had it revised. My breast milk supply went up a little but by the time it did at 8 weeks he didn’t seem to get enough still and would act hungry so I supplemented. Now we basically just comfort feed and do mostly bottle. But he still never takes more than 4oz at a time and that’s not every time. He goes maybe 4 hours between bottles most days. That is if he eats the whole 4 oz. I don’t know how to get him to eat more because a few times I’ve tried to make him take 5 and he ends up spitting up. Is it okay as long as he’s gaining weight? He still probably gets 6oz of breastmilk a day on top of the bottles. I don’t feel like I can make him eat but they’re really pushing that he should be taking more a sitting. How much do your little ones eat? And how do I start to maybe have him take more? Should I make him wait longer for food?