I'm sorry to hear about your dog. It could be a reaction to the preventic collar. Although reactions are rare with it, they do occur and some people have reported issues like this, especially if any got ingested (like a dog scratching at it and licking their feet, or licking something thy were lying against with the collar on).
To settle his stomach, start giving pepcid at a dose of 0.25 to 0.5 mg per pound of body weight. After a couple of hours from giving this, try giving a small amount of water and a very bland but tasty diet like boiled chicken, boiled hamburger, or gerber baby food (turkey or chicken are good). Only give about a teaspoon to a tablespoon (depending on how large your dog is). f he can keep it down, give another small amount in an hour. If ok, at his next meal, try giving the bland diet as a normal amount of food. After about 2 days, start gradually weaning off this diet to his normal food, slowly adding it back in. At this point, also stop the pepcid. It can safely be repeated every 12 hours through these first couple of days.
If for some reason he continues to vomit regardless or won't eat, he may need something stronger to settle his stomach, and a vet visit is in order.
Best of luck! Please let me know if I can help further.