Baby not gaining weight

Hi my baby was in 90th percentile when he’s 1 & 2 months old. 3 months onwards he’s been drinking very little and didn’t gain much weight. He’s coming to 4 months old now and he’s now around 50-75th percentile. Wondering any mummies experience the same situation whereby baby not gaining weight? His weight has been the same for 1 month from 3 months onwards, is that ok? Baby used to be able to drink up to 1100ml per day. Now average he drink only 450-580ml. He’s on both bm and formula #firsttimemom #firstmom #advicepls