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How old is your boy?
What kind of food does he normally eat?
Can you describe the licking behavior?
Thank you for the additional information. The licking of his lips could signify a couple of things. It could be that he has a bad tooth. This may account for his not wanting to eat dry food. It could be a sign of nausea. The straining episode you saw 2 weeks ago could have been the beginning of an IC (interstitial cystitis) flare, and may have progressed, now making him naseaus. He may have a foreign object stuck under his tongue or somewhere in his mouth, that he is trying to get rid off. Try to get a look in his mouth, and especially under his tongue to see if there may be something that does not belong. You can also look at his gums. Red gums around a tooth indicate gingivitis, or a more severe dental problem. Obviously, if you seeing anything abnormal this is a reason to see the vet. If you don't see anything, and the behavior doesn't resolve in a day or two, he should see the vet. He doesn't need any reason to aggravate his IC. Please let me know if you have further questions, or need more info.
I would definitely be concerned about a flare-up of the IC. Especially with the lethargy that you mention. If he is in a flare, and unable to urinate, this will definitely make him feel naueaus and lethargic, as I am sure you know. Please let me know how he is doing.
I applied Frontline plus on my mail 10 year old cat