After a thorough vet exam to rule out medical conditions, you may need to confine him. A large cage or very small room. until he regularly uses the box, and only the box. This is HARD!!! I know, I have had to do it. I lived with meowing 24/7 for a month because the cage was "killing" Jim. But it was necessary.

Also......if you have only 2 boxes and 4 cats, my best suggestion is at least 2 more litter boxes. And YES! you can find space for them because making a little space for a litter box is a whole lot better than cleaning up accidents.

Clean your boxes out with clorox. Honestly, cats are addicted to that stuff!

Buy yourself some Cat Attract litter. I understand it works almost too well. Someone told me how they tried this litter, and the minute they filled the box with it ALL the cats wanted to use the box. (LOL)

Is it possible that he is IN the box but the rear quarters are out of the box? (in other words, is the pee or poop close enough to the box for him to be "missing"? If so a larger box or a covered box may do the trick.