Poor baby
! I hope that he's okay!
I agree with the empty stomach thing, though usually vomitus from that is yellow and foamy. I'm so glad you're taking him to the vet, you just never know with these little ones.
Is there any possiblity that he ate something he shouldn't have, like some kind of object? If there is, ask the vet to do barium series x-rays right away. If you wanna know why, search my name and find my RIP post for my baby Starr
Big hugs to you both
!!!!!!!!!!