Soft spot?

So the other day, my sons dad said something about my sons soft spot being sunken and that he might not be getting enough breast milk. He’s 6 days old. I didn’t think it was but I’ve been watching it extremely close now and it does seem a little low.. I’m not sure what’s normal and what’s not though. I’m really freaking out.. how much is too much??? I’m thinking about a bottle of formula just to be safe.. but I know that’s a ball that if I start rolling it might not stop