could it be the transfer from colostrum to milk?

I had to avoid dairy, caffeine, and leafy greens. ❤️ if your milk just came in its likely he's getting an upset tummy from the stronger let down. Hope this helps a bit!

It could also just be trapped gas that's hurting his tummy.

It really depends on the baby. For my daughter I can eat anything I want as long as I don't eat out too much and I don't eat anything spicy and not too much carbonation. Start with the basics like chicken rice bread stuff like that then slowly add in things and that'll tell you what bothers baby's tummy