Should I be concerned?

My boy is not following the line for his weight, I am very by the book & when I see he isn't following what he should be, it breaks my heart. He is gaining weight but it doesn't look enough. He's very active. But he's not a big eater, he has around 3/4oz every 3 hours. He's 3 months old & im breastfeeding him. I just don't know what I can do to make him gain weight. I express a lot so I can see how much he's getting & he never finishes a bottle so I know it's not because I'm not offering him enough.