Does he have a thing about other critters, since his "encounter"?
That kind of thing changes some dogs.
One of my dogs is 10 years old, and has had bad hind knees since she was young--we try to keep her weight down and we don't walk her or else she also gets real stiff. My brother brought over his Rat Terrier, and of course my dog has to assert her alpha dog status, and spends all day chasing the Rat around--it usually takes a few days for her to recover...
We haven't gone back out into the wild yet, but will this summer.
It was funny, back in October I took a Firearms Safety course.
The class was on the second floor and at the top of stairs
there was a stuffed bear the spitting image of the one we encountered.
I asked the folks if it would be okay to bring Sam up to see it and they said sure.
He was doo-di-doo-doin along 'til he reach the landing half way up and saw the bear at the top.
He stopped, but as I reassured him he cautiously continued to come up.
When he reached it he kept his body back but stretched out his neck to give it a good sniffing.
He seemed to catch on that it wasn't a threat but was still a little wary.
The other dogs here can lay on their bellies with their hind legs stretched out behind them
but Sam has never done that, probably because of the hip joints.
When he playing or out in the bush he gives it his all
and it's not 'til later that the soreness and stiffness becomes apparent.
I have to make sure I pace him or he'll over do it.