A common cold is only contagious for the first 2-5 days after symptoms appear so he won't be neglected for too long.

I think it would be safe to say that you can let him run around and the such just don't handle him unless you have to. Picking him up to move him to & from his cage and for cleaning his cage should be fine. Myself & a lot of my friends have done so for years & years and never had a ferret catch a cold. Pretty much you just don't want to snuggle with him or handle him TOO often. And wash your hands before & after handling him.