Congrats!!!
Hopefully the marking will stop once he knows you, your home, and your dogs better.
For now, I would just work on stopping it. Watch him as close as you can like you're potty training all over again and whenever he begins to lift the leg say "AH-AH" real loud and make him stop. And since he was just neutered, it may take some time for all the testosorne to go away. I'm sure it will get better.
How awesome of you to rescue! I can't wait to see pics!