slow weight gain

I mentioned this in my last post, but I wanted to get some advice from those who have also experienced slow weight gain…
Finn is breastfed and had trouble gaining from the start (reflux, combined with inefficient suck and resulting low milk supply). It took him 4 weeks to gain back his birth weight. He then started gaining 1 oz/day, but this would fluctuate depending on how much he spit up.
I thought we were doing well…still gaining about 1 oz/day, but over the last month (he is now 4 months) he slowed down and only gained a pound in 4 weeks. I feel like this is due to the amount that he is spitting up (huge quantities). He doesn’t have a feeding aversion…although in the past few days with the gap in his meds I’ve noticed him taking in less with some aversion. we’re getting the prevacid capsules today.
My question is…. is this weight gain too slow? Do I need to head to the dr.? I’ve been weighing him at a LC/breastfeeding group.
thanks!