Hi there,
Not sure if this is helpful to you, but I spotted your post and it reminded me of when my son was around that age, so I thought I'd offer my opinion. My baby was also barely eating around that age, but gained weight, was generally happy, and all else was well. It became a struggle to feed him because he would refuse the bottle, and eventually, I started to dread feeding times cuz it always seemed to turn into a fight.
I spoke to his pediatrician about it. Like you, I was also stressing out because I thought he wasn't eating enough, etc. But this is what his doctor said: Babies will never starve themselves. If they're hungry, they will eat. And if they're not eating, they are not hungry. Then, he explained that all the guide books out there on baby care, and all the advice about how many ounces a baby SHOULD take at a feeding, and how many bottles a baby SHOULD take in a day -- well, these are all just guidelines. Some babies may follow the guidelines to a T. Other babies will eat a lot more. And then, some babies, like your's and mine, will eat less. A LOT less. So his pediatrician told me to do this: Cut back on his ounces. Basically, drop it down to the amount that he will ALWAYS finish, and offer that amount to him in every bottle. When the baby is able to finish every single bottle, then slowly increase it (by half ounce, or 1 ounce, is up to you to decide).
We went home and tried this immediately. We dropped every bottle down to 4 ounces. He went through a few bottles, and struggled through the others. A few days later, he polished off every single bottle. So I knew he was ready. I bumped it up to 5 ounces. Again, he went through a few bottles, and struggled with the others. A week or so later, he was able to finish the majority of his bottles... though, was not able to finish all. Eventually, we worked our way to 6 ounce bottles by the time he was 4 or 5 months old.
Overall, it was a slow process to get him to eat his food. But it reduced the stress on me... feeding times were no longer a fight every time... and baby was still growing, strong and healthy. Maybe you should give it a shot. I hope it helps you as it helped us.... :-)