Can't be sure that the vomiting is due to the surgery, but here is an article on vomiting post-surgery:
https://www.cuteness.com/article/pos...-vomiting-dogs
Some common causes are reaction to the anesthetic, and reaction to the pain killers given following the surgery. Or the vomiting might not have anything to do with the neutering. Monitor him closely, and if his situation doesn't improve, or if he doesn't regain his appetite by the end of the day, I'd definitely call the vet.