Baby not getting enough breast milk

I am currently exclusively breast feeding but I'm scared my baby might not be getting enough milk. He's currently 2 1/2 wks and seems to cluster feed constantly. I can only pump 3-4 ounces a day and have been freezing it. Today he has been nursing almost every hour for 10-20 minutes but I decided to try and give him a bottle of 2 ounces of my breast milk after a nursing feed and he drank it right down. He seemed really happy after the bottle and spit up a bit about 10 minutes later.

Any advice is appreciated!!!!