Help!

My 3 week old will only eat an ounce at a time sometimes and then will fight me when it comes to finishing it, he usually eats 2 ounces every three hours but as of the last few days it's either impossible to keep him awake past one ounce or he'll stop eating completely after one ounce. It's starting to worry me I know he should be eating more than that at this rate. We have an appt Friday for his month check up but I'm losing my mind idk what to do