I think a vet visit would be your best bet for figuring out the leg issue. The majority of senior cats most certainly have arthritis, but only a vet can say for sure if that's the case with your kitty. Norwegian Forest cats are also prone to cardiac problems, of which hind leg weakness can sometimes be a symptom.
Originally Posted by taitasdf
He also urinated outside the litter box which never happens (clear and plentiful).
This part caught my eye. Would definitely be time for some lab work (senior blood panel and a urinalysis). Dilute urine can be a sign of renal insufficiency and at 18, I'd be surprised if your cat didn't have some degree of compromised kidneys. Other common possibilities are diabetes or hyperthyroidism. All of these conditions can be managed with the right combo of meds, diet changes, and/or supplements.
So basically, off to the vet yee must go!