Please take him to the vet and tell the vet what he is doing. Peeing outside the litter box is the #1 symptom of urinary tract problems!
If the vet says he is okay, then consider making a few changes.
If you use covered litter boxes, take off the lid. Many cats refuse to use covered boxes.
Cats prefer fine grained unscented litter. So, you might try changing litter.
Make sure you have at least one litter box per cat.
Make sure you scoop the litter box daily, and with multiple cats, scoop twice daily. Cats often refuse to use dirty boxes.
If your litter boxes are real old, they may have absorbed odors even if you regularly clean them. So, try buying new boxes.
Make sure your litter boxes are in a place where the cat feels safe while going potty. If the other cat can sneak up on the cat while he's trying to urinate, he may be too afraid to use the box. So, move the litter box to a location where he can see the comings and goings of the other cats.
Put a litter box on each level (floor) of the house....especially if the other cat is blocking his access to his litter box.
Make sure you thoroughly clean all old urine spots on the rug and elsewhere. If he can smell the old urine he will think that place is a good place to pee. Use a flourescent black light to find old urine and treat all old spots with an enzymatic cleanser.
Try putting a plastic carpet runner upside down on the places he likes to pee....most cats don't like to walk on the "spikes" so they will avoid the covered spots.
Hmmmm. I'm sure there are other suggestions to try.
If all else fails, your vet can prescribe psychotropic medications that reduce inappropriate peeing, but try these simple solutions first so that you don't have to give him pills the rest of his life!