Baby won't eat😞

Kayley • 24 years old, I have a little boy who’s 4 and I’m TTC my second baby!! 👶🏻
My son is 2 weeks old and for the last day or so he is only drinking 2oz (60ml) per feed. He was tanking 5oz (150ml) the other day there, could my son have a sore belly or should i be concerned?