I am sorry Casey is not feeling well.
He may have a sore throat or be hypoglycemic.
Rub a drop of honey into his gums every 4 hours. It will sooth his throat and raise his blood sugar.
Drain the water from a can of tuna into a dish. See if he will lap it up. If he does offer some tuna.
If possible bring him in so you can make a steamroom in your bathroom by running a hot shower. Take him in for ten minute intervals to break up congestion.
Get a jar of chicken baby food with no onion powder. Warm it so he can easily smell it.
In addition, add one quarter of a 4 mg chlortrimeten to a tablespoon of baby food chicken or turkey with no onion or garlic.
Give it every 24 hours. It is very tiny so he will not detect it
Any of these are also acceptable:
Benadryl at a dose of 1/2 of a 25 mg tablet given twice daily or
Chlorpheniramine at a dose of 2-4 mg twice a day or
Claritin (Loratadine) at a dose of 2.5-5 mg/cat or
Zyrtec (cetirizine) 5 mg once or twice a day
It is important to ensure that the formulations used contain ONLY the antihistamine and are not combination products (e.g. Claritin-D contains pseudoephedrine, which could cause very significant adverse effects in a cat).
Please let me know how he is doing. Just reply here or if you prefer, we can speak on the phone.
Please be so kind as to rate my answer with positive feedback, as that is the only way I receive recognition from the site for my assistance. Thank you!
Providing a positive rating will not end our conversation, should you need me for any follow-up. Simply click 'Reply' for clarification or additional information, if needed and I will always be here for you and your kitty.