Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you today.
Now given the delay in signs and no other head trauma associated signs at this time, I don't feel that Scamp's vomiting is directly related to the antler hitting his head. Instead, we'd be more worried of GI upset form chewing that or a possible bacterial or viral gastroenteritis, or non-related parasites/protozoa infections.
Now as he sounds otherwise well in himself, we can try some home supportive care to try to settle his stomach. To start, if he hasn’t just vomited (since otherwise we’d need to rest his stomach for a few hours first), then you can consider treating him an OTC pet safe antacid [ie Pepcid (More Info/Dose @http://www.petplace.com/article/drug-library/library/over-the-counter/famotidine-pepcid), Tagamet (More Info/Dose Here @ http://www.petplace.com/article/drug-library/library/over-the-counter/cimetidine-hcl-tagamet)]. Whichever you choose, we’d give this 20 minutes before offering food to allow absorption. Of course, do double check with your vet if he has a known health issues or is on any medications you haven't mentioned. Though if he cannot keep it down due to nausea, that is usually a red flag that we need to bypass his mouth with injectable anti-vomiting medication from his vet.
Afterwards, you can consider starting an easily digestible diet like cooked white rice with boiled chicken, boiled white fish, cottage cheese, scrambled eggs, or meat baby food (as long as its garlic/onion free). There are also OTC vet diets that can be used (ie Hill’s I/D or Royal Canin’s sensitivity) too. The aim of these diets is that they will be better tolerated/absorbed by the compromised gut. Therefore, it should get more nutrients in and result in less GI upset. As long as improvement is being seen, I usually advise continuing this until signs are settled, and then weaning slowly back to your normal diet.
Finally, as dehydration is a risk for Scamp, we need to keep an eye on his hydration. To check that he isn't dehydrated, there are a few things you can test. Further to checking for gum moisture, do make sure he doesn’t have sunken eyes and that he doesn’t have a "skin tent" when you lift the skin. To see how to check these parameters for dehydration, you can find a good video HERE (http://www.ehow.com/video_12232503_dog-dehydrated.html). If you are seeing any signs of dehydration already, then that would be our cue to have him seen before this becomes an additional issue (especially as it is often dehydration that makes them feel unwell).
Overall, his signs do raise some concerns but the antler hitting his head isn't likely to blame at this stage. In any case, we’d want to start supportive care to settle his stomach. If he cannot keep that or water down at any point, appears dehydrated already, seems weak, or doesn’t respond to the above within 12-24 hours; then we'd want to get his vet involved. They can assess his hydration, rule out fever, make sure there is nothing in his stomach that shouldn't be there or any sinister viruses present. Depending on their findings, his vet can treat him with injectable anti-vomiting medication +/- antibiotics to settle his stomach, and get him back feeling like himself.
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