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There are not alot of useful over the counter medications in cats and many can be toxic. If this happens to be related to an allergy, it could respond to Benedryl (diphenhydramine) at a dose of 1 mg/lb twice daily. It comes in a liquid form that has 12.5 mg/ teaspoon. Just make sure that diphenhydramine is the only active ingredient. Otherwise, if this sneezing is related to a viral or bacterial infection, nasal polyp or tumor, or other illness, your vet will need to narrow down some of these issues before beginning treatment. Because of his age, dehydration is a concern, you can maintain hydration and nutrition with a 50/50 mix of pedialyte and water if he happens to go off food and water. I hope this helps and gives you direction.