help please my baby won't keep down formula

So my baby is now 3 weeks old, he was born 5lbs 7.8oz, at his last checkup he was 5lbs 13oz. He won't keep down any formula we've been using enfamil and similac samples. We switched over completely to enfamil because he seemed to like it better as well as he wouldn't keep down the similac, but we just fed him the enfamil and he spit it all up. This has been going on the past 3 days. I don't know if he's eating to much, if it's the formula, or acid reflux. He'll eat like 1oz but if he eats more than that he spits it all up. Any insight would be helpful. Please no rude comments.