It's time for a trip to the vet to find out what's happening. At 9, your pom is a senior and it may be something relatively simple like osteoarthritis. It may also be something more serious like a growth interfering with movement of the jaw joints. There's no point waiting and speculating - it's time for professional help so call and make that appointment today!
IPossibly his teeth are bad, and closing his mouth puts pressure on them and it hurts, so he tries not to.
Apparently it's not impossible for a dog to get a dislocated jaw. And after the first time it happens, it can recur if the mouth is opened very widely later in life.
Do you perhaps recall any time in his past when he had a jaw dislocation? Or maybe it could have happened before you got him, when he was younger?
But obviously if you have no memory of this, and he has been with you all of his life, then maybe not.