Sorry to hear you're seeing the changes; I am seeing changes in Shadoe the last few days and so I know it's upsetting, even if you recognize it.
I am sure you know him better than you know yourself, so if there are any oddities that truly seem out of place from his normal abnormalities, what about having him get a checkup?
What's your gut tell you? I normally go with mine, and it either reassures me I was seeing nothing or confirms my suspicions.
Are there any signs that remind of some past issues? After seeing Shadoe with p-titis attack, I watch her like a hawk to the point I am sure I am annoying.
Any changes you can make to routine, or food or something else to test him?
Maybe step back and make a list of things to check to eliminate this or that. Also consider anything that's changed recently that may be a cause.
Does he ever had bad days where he feels just under the weather and then comes around later?
I don't have enough knowledge to offer more than just generalities, but I hope he comes around soon from whatever is the problem.