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Any problems meowing?
Dos he sound congested when resting?
Any nasal discharge or coughing?
Thank you for the extra information.
It is definitely worth having checked out by your vet.
It could be something as simple as a respiratory infection or sinus infection. It could also be from polyps in the nose, throat or ears as well. If he is over weight, that too can contribute to the problem.
Keep a vaporizer in his common sleeping areas if possible. Also using the steam from your shower can help break up and mucus congestion in his head or chest. Just put him the bathroom for 15-20 minutes at a time to breath in the steam.
But do get him checked out to be safe.