As far as I know, myositis can wreak havoc on the immune system and bring on secondary infections. I don’t know much about it either, but the atrophy in his head muscles doesn’t look good. If he’s walking strangly and getting tired easily like you said, he's very likely atrophying bad in his core and leg muscles as well.
Sorry to be a downer. I would get a second opinion, especially if you feel the first vet is hesitant with him and hasn’t done his homework.
I guess that would explain it then cause he's always been healthy till he got it

And nah, not a downer. It's okay. I've known he's been getting worse but I do feel kind of bad for not forcing him to exercise more all this time or not doing PT or something more for him

But yeah, I think we definitely need to make an appointment with anothee vet. We have one we take the cat to and they're great.