
In an ideal world both dog and bitch should be fully mature before being neutered, especially if they're of larger breeds. However having both male and female in the same house spells incredible problems, even for people who're used it!
Usually I'd suggest waiting until after the bitch has had her first season (midway between seasons is easiest and safest) but she could come into season at any time and your dog would think all his Christmases had come at once! Even if he was castrated tomorrow he wouldn't be guaranteed sterile for some weeks, and the result could easily be puppies.
So if I were you I'd get them both neutered asap - preferably tomorrow.