He is so cute!!!
You might consider using different litter...the finer the better.
It is all about "feel" they have to dig in the litter and then cover it.
Takes more time than just a quick squat and pee.
So, perhaps it is the texture or the odor.
I use Fresh Step and muy guys are good with their habits.
I use fresh step as well, and we have three electric litter boxes for four cats. I LOVE the electric ones, and I am guessing my cats do too, no mess for them to step in.