How big is the corner box? There are some that are small and other that are larger. The small ones are too small for 90% of rabbits. The large ones are better, but still not great. A rectangular cat box is better and most rabbits like them better. A large size will work for most rabbits.
Putting hay in the box (a hay rack is good for this) can help to get them to use the box. If they have to get in the box to eat the hay, then they will usually pee and poop in the box.
Cleaning up the pee put of the box with a paper towel and putting that in the box can help them to get the idea that they are supposed to pee in the box.
I am not sure what kind of litter you are using. As long as it is non clumping, not clay based and not scented, it should be ok for now. The best litters are either wood pellets (horse stall bedding or wood stove pellets) or recycled paper pellets (Yesterdays News). Both are safe for rabbits and really good at absorbing odor and liquid. The wood pellets are also cheap, about $4-7 for 40 pounds.
Young rabbits and intact rabbits can be harder to litter train. Getting him neutered when he is old enough should help. As he gets older, he should get better at it.
