Baby not gaining

So baby has gerd and milk allergy, calling ped today just want to see if anyone has experienced this. He is not gaining weight he was born in the 70% percentile and fell to the 16th percentile. I weighed him yesterday and he’s in the 12th percentile now. We put him on omeprazole and he’s drinking similac alimentum. Before the changes he was drinking 15-18oz a day now he drinks 22-26oz but he is not gaining weight, has anyone experienced this and do you know what the doctor will recommend? I’ve never had a problem with babies chunking up and am starting to worry there is a bigger issue going on with him. Any advice welcome, thank you!

Update: pediatrician upped his dosage of omeprazole and we have a weight check next week, if the omeprazole doesn’t help they will do an ultrasound on his tummy to see if has some rare esophagus condition