Gaining enough?

Someone tell me that this happened to their baby as well and I shouldn’t worry to much. My son is 4 months and 3 weeks. He was born at 7 pounds and 13 ounces. 65th percentile. He is now 13 pounds and 7 ounces at the 4th percentile. He’s eating enough during the day sometimes over 32 oz. the longest he been without gaining any weight was 6 days and even lost weight, 4 oz in a week. The first two months I pumped but I figured I wasn’t taking in enough calories to feed my baby so we switched him to formula. He stayed at the 5th percentile for a month and a half and now he is falling again. I’m not sure what to to think of it. I’ve sensed something was wrong when he had to stay in the hospital two extra days when he dropped a little over a pound. His pediatrician doesn’t was to run tests until he’s at the 3rd percentile. I don’t know what the problem would be, it doesn’t even make any sense at all. I just want him to be ok.