My almost 5 month old not eating a lot

Michele

This started about a little over a week ago he used to eat 32 or more oz in a day but now hes only drinking 21-28 oz in a day I took him to the pediatrician they dont know why hes not eating as much he also lost a little weight on 1-28 he was 14 pounds and on 2-5 he was 13 pounds 15 ounces. Any ideas or suggestions

Ps I tried switching bottles and nipples and giving less formula at a time