Hard to believe your boy is already 11! I remember when you adopted him!
I'm with MaxaLisa--we'd likely run a tick panel and do an antibody thyroid test. If it turns out to be either, then you've got another treatment to try--and if both are negative, then at least you know what it isn't
and you know that loving Leo and carrying on is probably your best option. Give the sweet boy a hug from me, pls!