Way too cute!!!! Just love those eyes. All you can do is continue to reinforce the litter box by placing him in it particularly after you notice him eating or drinking. He still looks pretty young also so maybe even supplement him with a little bit of KMR in his food or seperately. My recent rescue did similar and I just kept a very close watch soon as he took a drink or ate a few bites, into the litter box he was placed. He soon got the idea. But very important be sure you have more than one box or place it in the area he spends most time in and definitely use a solution that will not cover up the accidents but remove the odor or he will use it again. When you cant watch him such as when your at work or such maybe a kitty cage with the box inside or the bathroom with the door closed. They will not usually pottie where they have to lay or eat even as a kitten.