@A Peacock called Penelope
The cage is kinda small but for a cage its large. Cages are just small in general.
Personally, I would maybe try to build something similar to this to keep him him. Something over the top too so predators can't get to him. It doesn't have to be this big. About 7 feet long and 2 feet high would be fine, but the bigger the better. Also, a solid ground would be good. If it over dort or grass predators can dig under it. If you have a paved area of even a large board or some tile all under it would do.
All you would need is some chicken wire (or hardware cloth), and wood. If you wanted you could attach small hinges to make it into different shapes but one like the second picture would work two.
Good thing about this is you can use it inside too if you end up doing that.
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As for litter training, have you noticed him using a certain corner? If so, it's best to put the litter box there. Any coco puffs you see outside of it should be put in the litter box. Also, lots of hay or grass in the litter box will help so much. Rabbits like to eat while doing their business so this will entice him to use it.
The reason he's not very active when go first get done with school is because they are crepuscular. Most active in the early morning and late evenings. Eventually he'll get used to our time. If you can, I would let him put before school starts, check on him after, and give him 3 hours of playtime outside of the cage.
Have you tried grooming him yet? A dog flea comb works best but even just a regular comb will do. Grooming is important because they can ingest fur while grooming themselves which can block up their system and they could.go into GI stasis and die within 24 hours.