help! male dog issue

Never had a male before, so don't know how to prevent this situation. Sammy, our new foster, marked in the house this evening. DH caught him in the act and basically interrupted him, but....how do we teach him this is not acceptable?

A little background-he is approximately 6 years old, he is neutered, and apparently has spent very little time inside. We had him on a leash the first 2 days, and there have been no problems, so we let the leash off this evening.

Any suggestions? It might be hard to find a home for him unless we can stop this behavior, and he's such a sweet, sweet boy.