HI and Welcome to Pet Talk
How many boxes do you have? It could be that one of the other cats are in the box when he wants it and he just can't or won't wait. Also he could be one of those kitties like my Vixen. Vixen has potty box issues. We have 2 boxes but to Vixen one is for peepee and the other for poopie. If my other cat Lilith goes poopie in the peepee box Vixen won't use it and if Lilith goes peepee in the poopie box then Vixen won't use that one either.

But there could be other issues. Maybe the new kittie is getting picked on more than you realize or maybe he is not happy in the current situation. This could be a behavioral problem. I am not too sure but I am possible one of our more experienced PTers will give their solid advice. Look for posts from Queenscoopsalot, kimlovescats, jenluckenbach and there are others. Those 3 know a lot.

Good Luck Michelle