sometimes older cats get too tired to go to their box, too lazy, or probably now it's just a habit and cats can smell the urine even if the bed has been washed, even though we cannot. break his habit. and put another scent there that he KNOWS: DRY CAT FOOD IN A SMALL PLASTIC BOWL THAT WON'T TIP OVER. the only way i broke a habit like that before was i placed the cat's food (i used dry food) in a bowl and placed it where the cat was urinating that i didn't want him to be (at that time it was my kitchen floor in front of my fridge!!). cats don't normally go the bathroom where their food is, so that might work with his bed. cats operate on smell and the smell of his food should alert him that the bed is not for peeing. just put a bowl of dry food on top of where he pees and if that is where he lays, then just push the bowl as close to it as possible so he can still lay down.
I've used the bowl of dry food trick and it always works. but only if the cat has a clean litter box in a convenient location where he doesn't have to feel stressed to get to, e.g., doesn't have to cross any dogs or loud noises or such. right now it seems the cat thinks his bed is a sort of litter box. i wonder if he has blanket or towel in his bed? sometimes the extra material they seem to feel is like a litter box, like towels or rugs they can scrunch up like they're burying their urine in a cat box.
if the cat still pees in the bed with the bowl of dry food then you know it's a medical problem and not a habit! it would mean the cat feels the urgency and just can't make it to the litter box then.