Breastfed baby went two weeks without weight gain?

Kelsey
He was 7.1 at birth. Dropped down to 6.11 4days after. By week two he was 8.1. Which is amazing right? Well, then by week four he was STILL at 8.1. No weight gain in two weeks?? He got an inch taller but that's it. He has reflux so doctor gave zantac. Said maybe he was spitting up, too much to gain weight. What do you think? Anyone else? How to get weight up?