Clavamox can be very hard on the stomach and make many cats nauseous, esp. if it's given on an empty stomach. When you speak to your vet tomorrow, ask if you can switch him to amoxicillin, which generally has fewer gastric side effects for most cats.
It is very important that he eat, so you may need to assist feed him if he won't take food on his own. The following links will help with assist feeding:
It sounds like he may be adequately hydrated at the moment, but if he doesn't start drinking tomorrow, you should ask your vet to teach you how to administer subQ fluids at home to prevent him from becoming dehydrated again. I can provide you with links to online tutorials for administering subQ fluids if you'd like to acquaint yourself with the procedure. It's really not difficult, and it can definitely be life-saving.
There are appetite stimulants that can be given to try to help get him eating again. Cyproheptadine worked well for my Billy at a dose of 1/8 pill once or twice daily. Mirtazapine is another drug used for appetite stimulation in cats, though it's important to start it at a low dose of 1/8 pill once every 3 days (in spite of the fact that most vets prescribe it at a higher dose). Higher doses cause some extremely unpleasant side effects in some cats, so it's always safer to start at the lower dose and increase slightly only if the lower dose is not effective.
If Sickey's blood glucose was elevated in his bloodwork, it'd be advisable to request a fructosamine test to check for diabetes. If his blood glucose was in normal range, then you needn't worry about diabetes.
Rapid weight loss and excessive thirst are also symptoms of hyperthyroidism, though that is typically (but not always) accompanied by ravenous appetite. It's not common for such a young cat to develop hyperT, but it's not unheard of, either. It wouldn't hurt to run a Total T4 to check his thyroid function, just to rule it out.