21 pounds! Yikes! Big boy. That in itself may be part of the issue. If he is that big it is possible he is just not comfortable in a normal sized kitty litter box. My suggestion would be to try one of those under the bed storage boxes. Not too high sided so he can get in and out of it comfortably.
I think you had mentioned before that two of your boxes are covered. So between the covered boxes and possibly being too small for him to be comfortable in it could lead to his not being a happy camper and peeing where he is most comfortable.... your couch or bed.
I know you said hubby doesn't like the uncovered ones but hubby isn't the one having to use it. I would think if it helped to stop the problem then hubby should just learn how to put up with an uncovered large box.
