Well, he's gonna eat it if you don't give him a choice in the matter. Just leave a bowl of the food out and refuse to give him anything else until he finishes it. No dog is going to starve himself to death if there's food available.

Maybe try to make him some vegi dog food yourself?

Make your own dog food, with a bit of research you can the right food which has all the nutrients they need, and it'll be much cheaper than buying it. Unlike cats, dogs are omnivores like us so they absorb the nutrients directly from plants so don't need expensive processed foods, just need the right plants

You've probably already tried this, but our dog, who was allergic to many things, was still able to eat horse meat. If you haven't tried this out yet, maybe it would work? Though I think it was also pretty expensive (my parents' dog, never paid for it myself). Otherwise, I agree with what some others here said. Just give it time, your dog will eventually get hungry and eat the food. Be careful with trying any home-cooked veggie dog food versions, they might lack some essential nutrients (essential amino acids, B vitamins, other things - I'm not familiar with the exact dog physiology) and be very bad for your dog in the long run. If you do decide to try it, do a lot of research beforehand and talk to your vet.

Natural Balance vegetarian formula works for alot of dogs allergic to animal protein. giant bag is around $60 Nature's Recipe is easier to digest and maybe more flavor also vegetarian. big bag $50. Halo has a full vegan formula, $60. wet and dry is available for all three brands

There are vet prescribed diets with hydrolized protein so that they can't have an allergic reaction. Its a common food animals are put on that have food sensitivites. It's still a little more expensive than a general commercial brand dog food, but it could be a bit cheaper than 90 dollars a bag, depending on size.

Take him to another vet to test fir allergies. Don't mention what the last one said. See if he is actually allergic to anything.

My dog is an extremely picky eater and she just loves rice. I don't give it to her as a meal everyday, but it's a good option to give to her once and awhile.

My dog has the same issue, mix his food with a cup of rice and he will eat it! :) Best of luck, OP.

Now all the other dogs are gonna know he's vegan

Add some olive oil, my dog has the same problem.

try mixing some rice in with the food