When we got Keoki he was box-trained. I didn't continue with that because I'd rather drag a dog kicking and screaming into the rain than to have the dog potty anywhere in my house, LOL, but that's just me
{I have two cats, and am SO SICK of their litter boxes too!}
I suppose, if we'd simply put a box out and showed him where it was, he'd be box trained to this day.
I have a friend w/a box trained toy yorkie, so I know it can be done.
If your litter box is large enough for him to comfortably stand, then I imagine it'd be just a matter of taking him to it and telling him to potty – same as housetraining w/out the door! Of course, there may be the whole "lift the leg" issue for a boy . . how you'd keep that in the box, I don't know.
Good luck!