When is a male kitten capable of reproducing?

I have a 10 month old female cat who went into heat for the first time months ago, and a (newly) 7 month old male kitten.  They're both scheduled to be spayed and neutered in 3 weeks (that's the earliest date I can get).  Am I right to worry that he might get her pregnant?  Should I be separating them until then?  

Yes, he can definitely get her pregnant. She can get pregnant at 4 months. You have two options:

Separate them until surgery. This may or may not be easy.

If she is pregnant at surgery, you could opt to abort the fetuses.