Issues gaining weight?

Ok so my lil man is 4 months old. He's hitting all the milestones but apparently isn't gaining/growing enough. The ped stated 20min on the breast then 3-4oz formula and 6oz when I'm at work. First I'm a bit freaked that he's not growing as he should and apparently my milk isn't helping like it should. He's been 1/2 and 1/2 since day one but now they're pushing formula more because it'll give him the caloric intake needed? I'm fine with that, upset I can't provide as well as needed but as long as he gets back on track I can deal. My biggest worry is he'd been asking for more so we'd do 5 oz ( or longer on boob if I was home) but occasionally he'd volcano it back up like he'd over fed. So now I'm worried pushing him to 6 oz straight out he'll throw up a LOT. Ugh this while situation sucks and I'm worried about my little man probably more than I really should be since the doc was pretty calm and scheduled for a month out to recheck. Sorry just needed to vent a little.