First thing, believe me, I know how frustrating this is. Couple of questions.
Have you been to the vet? And especially mentioned that all was well til you moved? Truly, that is the first thing you want to do - rule out any medical issues first. If, let's say, your kitty developed a UTI, he won't want to use the box because he probably associates the box with the peeing hurting him.
A lot of times cats develop urinary and other issues simply due to stress, and if moving is stressful for humans, it's much worse for cats, who tend to be territorial. Changed the type of litter? Is the litter box location more or less the same place as in the old house? Sometimes as cats age and develop arthritis, it's difficult to climb stairs in time to go in the box.
So, let's say the vet gives your cat a completely clean bill of health, after testing urine and feces. What I did in a similar situation (including crying myself), was:
Add at least one new litter box (where are the current boxes?). The rule of thumb is one box per kitty, plus one. I actually have four boxes in different locations for two cats.
Clean twice a day, if you haven't been already.
Clean the accident areas with an enzyme cleaner - otherwise, the area smells as though it's an approved place to go potty.
I've had very good luck with Cat Attract litter (you can get at Petsmart and some other big box stores). There's a booklet inside the bag that discusses dealing with litter box issues which is very helpful, and check Dr. Elsey's website as well for advice.
I've also read some people will set up a few boxes with different types of litter, just to see if a certain texture is preferred by the cat. And yes, cats (like humans) can change their tastes as they get older.
But, if you haven't been to the vet, please consider getting an appointment for a full physical (and senior panel, if your boy hasn't had one before) as quickly as possible. Tell the office it's regarding litter box issues and runny poo - they'll tend to get you in very quickly.
Good luck, believe me I know how frustrating it is (my prior owner put in white carpeting!) - I honestly did cry a few times, but my vet and shelter kept helping me with advice and now my boy always uses the box, unless he's ill (and then I race to the vet!)
