Has he tried putting more than one litter box almost side by side.
What he can do also, is get some dirt from outside and put it in the litter box with some of the litter. Once the kitten keeps going in the litter box, put less dirt and more litter each time. He might also have to keep the kitten in one room with at least two letter boxes, to retrain it to use the them. Once this is established, then the kitten can have free roam of the whole apartment again. I wish your brother luck.

I have three little boxes side by side as my guys will pee in one and then hop into another to make a poop. Plus I have three more boxes around the house.
Mind you I have 6 cats that use them.