Assuming he's doing it outside the box, clean the area he went in and put a small litter box in every place.
He's still young and sometimes they don't have time to make it to the box so you'll need to have a few boxes out for a while. He needs time to adjust to his new surroundings.
Well, I put that into consideration. He goes the same place each time. Right beneath my headboard. I tried placing a box there, but then he moved to my closet. So, the little box is back in its orginal position and he's back to using it beneath my headboard.