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Hello, I'm sorry to hear about Tino.
Hives are related to an allergic reaction, usually to a bug bite or sting.
You can give Benedryl (diphenhydramine) at 1mg per pound orally every 8 hours for 72 hours to allow the allergen to clear. That's one 25mg tablet per 25 pound dog or 1/2 of a 25mg tablet per 10 to 12 pound dog every 8 hours for 3 days.
Antihistamines can cause drowsiness or hyperactivity but these side effects wane with repeated use. They are very safe but must be used with caution in dogs with heart disease or glaucoma.
If your fellow is vomiting or has diarrhea or difficulty breathing then he likely needs to be seen at an emergency clinic for a steroid injection as that indicates his reaction is worsening.
Please let me know if you have any further questions.