I have a few questions with the litter box issue you are having.
What kind of litter do you use (what brand also)?
What kind of litter box do you use?
Where is the litter box situated in your house?
Has there been any incidents that would have over-stressed him lately (anything that even doesn't seem to make sense can be very helpful in this, no matter how small.)?
For some reason litter box issues seem to be my strong point. my uncle(who is 83) had a 14 year old cat stop using her litter box, a switch of the box type and a change of litter type and she was back to the box. she didn't like the tall back on her old box and the clay litter started getting stuck in her paws and due to her age she had issues getting it out.
- I should also add here i did go through school for it and worked in the veterinary industry ;)
how many litter boxes do you have? The general rule of thumb for multi cat houses is litter boxes = cats +1, to make sure there is always a safe and empty box and cats can't intimidate one another into not using the box. signs that we see aas nothing cats can take to be very stressful - a dog watching while he does his business may be enough to make him uncomfortable.
If you only have one or two boxes, I suggest trying to get another for a total of 3-4 scattered throughout the house. Yes it is inconvenient to have litter boxes everywhere, but it's even less convenient to have messes everywhere, and once kitty finds a safe spot and routine, he may stop using certain boxes and you can remove them. If you can, put at least one of the boxes in a place that is quiet and hard for the dogs to reach.