It might not be big enough and if you think it is big enough then i would get a litter box that has big sides and a roof so he wouldn't be able to go to the bathroom on the floor...there's nothing wrong with it i think he just doesn't have good aim or like i said it might be to small...
Your Cat might not like the type of litter you use. It has been known for cats to dislike their litter. Try using Nature's Best, or shredded newspaper. He might not like how it feels on his paws. But always talk with your Vet for the best advice and approach on the issue. Hope all goes well.