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
wonder if there's anything to boost his immune system? And that makes sense, he mostly lays around but he's always been pretty lazy but I definitely used to do more with him and at least try to walk him and I really haven't been because he's been so bad lately but that probably makes him worse because he's not moving so it's kind of a vicious cycle. He's also always had a bit of a sway but I definitely think it's worse now. Do you think the myositis is making him worse/weaker or is he just fat/overweight, out of shape, and older? It's hard to tell. :/ I used to do some rear end awareness stuff. Might start up again. Couldn't hurt I guess. 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.
I hardly call it camping but yeah we're at jellystone. These kids opened and shutdown the pool/ splash area yesterday so I was literally in or by the pool all day yesterday and had plenty of time to chime in. Today probably won't be any different. It's all they ever want to do when we come. 
