Thanks so much for your very helpful reply. If his thyroid count is up his dose of medication should be monitored and reduced, if necessary.Many times when hyperthyroid is treated, renal disease worsens so his kidney function needs to be evaluated.
HCM is another area of concern with a hyperthyroid cat. There is a cheek swab that can diagnose or rule out that disease.
Here is information on both:
Some of these patients get pneumonia. With the symptoms you list I would check for that first thing.
Eating well is a great sign because a cats appetite is the best barometer of his health.
Since you want to go away and he may need to be hospitalized it would be a good idea to board him at his vet until you come back.That way he will get treatment and you will not need to cancel your trip.
Please let me know what happens. I hope you boy will be fine
Have a wonderful holiday,