This could be from intestinal parasites such as worms, coccidia, or giardia or it could be from ingesting something he shouldn't have such as a toy, a sock, a rock, a coin etc.
If he is not fully completed his puppy shots potentially he could be showing signs of a viral disease such as corona or parvovirus or distemper.
If this is parvo then you should talk to your vet about using the human med Tamiflu which you can read about here
www.marvistavet.com/html/body_oseltamivir.html
You can try giving the pup plain pedialyte instead of water and bland food such as boiled chicken meat or hamburger with the fat drained off mixed with plain cooked white rice instead of his regular food.
The pup could have a sensitivity to an ingredient in his food too and that also can cause regurgitation usually right after eating.
If there are any signs of blood or what looks like digested coffee grounds in the vomit suspect internal bleeding.
If he is not passing stool suspect possible blockage of the intestines.
You should have him checked by the vet but you are the one who can see how sick he is so you have to decide if he needs and emergency vet tonight or to see your vet tomorrow.
Hope this helps you!

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