Higher sides for sure. The top entry boxes work the best for us. If he’s only going once or twice a day you may want to change up his water dishes so he’s drinking more. I have a picky kitten and I got one of the little fountain drinkers for him and he’s drinking much more now.. just a thought.
If he's in the box when he does it you'll need something with higher sides my female cat patches used to do that she would get in the litter box go right up to the edge and pee and it would just shoot right over the top of the box so either something with higher sides or an actual cover.