First, I would like to thank you for deciding to rescue! It is very rewarding and I can assure you that you will not be sorry.
I would say to ask a lot of questions when you get to meet his foster mom at Petco this weekend. Don't expect much in the way of interaction because of the setting but just let the 2 kind of feel each other out and see how it goes. There is no one answer for every dog. It is entirely possible that after nuetering he will no longer mark but it is no gurantee.
My suggestion would be to make a nice long list of questions to take along with you when you meet him. Take a friend or family member along as well so that you can spend time alone with him as well and consentrate on your opportunity to speak one on one with his foster mom. Here are a few that I can think of off the top of my head:
1. How does he interact with his other foster brother and sisters now?
2. How long has he been in foster?
3. How long has he been in this foster home?
4. Does he mark?
5. Will he wear a belly band?
Hope this was helpful.