Is there a reason the vet thinks this is more than just age-related?
7+ years not young... but if this came on suddenly, that would be more of a concern than something you've seen gradually come on.
I'd be poking and massaging all over to see if there are any sore/sensitive spots that you can point to.
Hips are always something I worry about (warranted or not). Maybe a back problem? Could be he just slept funny and got achy?... does he have his normal bedding at the inlaws, or does he sleep on something different than when at home?
Do the inlaws get him out for the same amount of exercise that you do when home?
How about food... any extras they might have given him that he maybe over-indulged in?
Any other symptoms? Pupils dilated or a 'hard' look in the eye? (like he's dealing with pain, but won't admit it)
Does he still get the same excitement around his favorite items (toys, food, people)?
I'd track his actions for a few days and see if you can put a schedule to the problem... or if it goes away now that his pack is all home again...
Just throwing out some thoughts... if you can track it down to what might have been different, maybe you can figure it out without all the medical procedures.
But if things don't improve or seem to be getting worse... I'd jump into all the tests... but that's based on some ugly still-too-recent personal experiences having to do with cancer tumors - and I think it's too early to assume the worst for Joe - so DON'T!
I'd take AB's advice and get some joint supplements, too.
