You are truly a wonderful friend taking in Carol's little guy while she is recovering from her stroke. I'm sure they miss each other terribly but at least she knows he is safe with you.
Just keep doing what you are doing. He just needs to adjust to his new enviornment. Did he mark when he was living in Carol's home or are you just concerned because he is now in your home? If he wasn't marking before he should be just fine. But if you do see him marking then you need to use a belly band immediately. When I brought Bogey home he peed all over my kitchen. I thought I was going to lose my mind. Now he is the best of the 3
If you need ANYTHING just shoot me a PM and I will help you as best as I can. BTW his picture with your friend is adorable. They look like they really love each other.