Litter Training

Thank you for your input regarding bonding my neutered male mini rex and my unspayed female "mutt". I have decided to keep them separate for a few months until I get her spayed.

My question now is regarding Buck's potty habits since getting her. he is about 5 months old and has been spayed for a month or so. He took on to litter training really fast and was great about it until I got Molly. Now, he pees in his pen and poos non-stop EVERYWHERE. The poo, I am not so concerned about, I know he is doing it for territorial reasons that it is easy to pick up. Regarding his pee. He does not pee when he is out and about, only when he is in his pen. I deodorize his area and cleaned it to neutralize the urine smell. I know that he is trying to mark his territory all over again and that having a female in the same general area is the reason he is doing this.

Is there a good way to break him of this habit and encourage him to use his litter box at all times again? He does still use it, just not consistently like he used to.