4 month old doesn’t eat!!!! Please help

I don’t know what to do. He will go 12 hours without drinking milk (formula) in the day. I’ve tried so many different kinds. He’s only 13 pounds and only gained 0.5 pounds in one month. My doctor isn’t helping and keeps saying he’s healthy. He only has 2-3 diapers in a day and they’re not even heavy. They’re still cruncy. Doc keeps saying “healthy” because he’s gaining SOME type of weight even it its only .5. I know doctor is always right but seriously my son only eats 10 oz in one day as a 4 month old and they’re all in the night time forced while he’s sleeping. I’m going mental here. Please what can I do?