Is there someone in the tank he could have been fighting (LipLocking) with?
I had this happen to one of my FH. He had Lockjaw for at least 4 hours, or 4 days......I forget. I know it was a while though, cuz I was freaking out. It eventually went back into place. I have heard though, sometimes it doesnt. If that happens, you could guide it back into place, but that is VERY iffy.
PM a mod to move this to the disease section. Lupin should have a say in this. The swollen part is odd. Doesnt look right. Might not be lockjaw. I'm 50/50