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I am so sorry he is unwell and appreciate it must be upsetting for you. Unfortunately, it is a vague and non-specific sign and may indicate a significant issue or a more simple issue. Certainly, given that it is out of character for him I would suggest there is definitely an issue. Panting can often be associated with pain, this pain can be located anywhere in the body from a joint pain/back pain to anal gland impaction to more significant internal disease. The priority is that he is alert, responsive and not collapsed or gasping for air. He should have nice salmon pink gums and not be vomiting or have diarrhoea. I cannot suggest any home remedies to settle him as they may mask the progression of a more significant issue. There are two options for you, if you do not think he will settle and you are going to worry and have an unsettled night then perhaps it is best to get him checked at the ER.
If he is likely to settle and not displaying any of the more critical abnormalities mentioned above, you could get him checked in the morning, but if he were to worsen or new signs develop, get him seen sooner.
I am sorry this is vague advice, but I am sure he will be fine