Update on 21 y/o Simon - he’s doing amazing!
Blood tests all clear, thyroid levels normal, he is back to eating at full speed, pooping and peeing normally! He must have just had a stomach bug, or it was brought on by stress because we went out of town 2 weekends ago. He is 100% back to his old self and doing amazing.
His vet said I did the right thing by stopping the clavamox based on the side effects I was seeing, and that he would not have prescribed it (would have done metronidazole instead). However now he does not need anything because he’s doing great. I swear this cat might outlive me. Thanks for all of your comments, suggestions, and support!