Given his age I would be looking at age related conditions to rule out, navicular is certainly one but also things like Cushings or a tumour pressuring somewhere it shouldn't (benign or otherwise). It could also just be grumpy old man with aches and pains and less tolerance, or a general injury, but it sadly sounds like more than that, so hope the vet can do blood tests and rule a few things out for peace of mind unless the problem is obvious to her from an initial exam.
Teeth issues would be my other main consideration, particularly if not checked for a while. Teeth grow at an angle and develop hooks that need more rasping in older horses to keep right - particularly right at the back. And you can get cracks and other damage which would definitely give him a good reason to behave badly. If a bad tooth is removed, it may make an almost instant change in his outlook, within a couple of days if that proves to be the issue.
Good luck and hope to hear more when your vet has been.