What's his litter box set up like? Big box, little corner box? Do you use pelleted litter and do you put a soft layer of hay over the litter?
Rabbits don't necessarily poop constantly. They can hold it in and chose when they want to poop. So there will be times, especially the middle of the day when he's more tired, that he may not need to use the litter box as much. You can gauge how a rabbit is doing in large part, by how their poop looks. If it's a good size, normal round shape, and crumbles easily when broken apart with your fingers, then your rabbits digestion and diet is likely just right.
https://bunssb.org/bunnies/guide-bunny-poops/
With the litter training, keep in mind you're dealing with a hormonal unneutered rabbit. So the fact that he's using the litter box at least part of the time is pretty good. Some unneutered bucks can be real pigs and a complete mess until they get the snip. If you're there with him and it looks like he's going to pee or poop outside the box, you can always usher his little bunny bum into the litter box, so he knows not to do that. Doesn't always work, but for some rabbits it will.