Pumping almost nothing. How do I know if bubs is getting anything while nursing?

Hello ladies!

My little man is just gone 3 weeks old and to begin with I was breastfeeding and pumping and I was usually getting 3-4 oz each side twice a day from pumping.

Baby is now cluster feeding and every time I have tried to pump in the last 3 days I'm getting less than half an ounce from both sides combined. We've started supplementing with formula as after an hour or so of continuous nursing he is still showing signs of being hungry and my nipples are just too sore to keep going.

Does this mean my milk is drying up? If I keep nursing him will it come back? And how do I know if there is any milk at all when he is nursing? I'm sure I can hear him swallowing but I have no way to know how much he's getting