That is encouraging news Nancy. A few questions for you so that maybe I can relate this to my situation:

Was the recovery sudden? I have heard people just waking up one morning, and its gone.

Did exercise trigger the symptoms? Sounds like this is not an issue any more if he is running cross country.

Did you try any treatments - medication, neurofeedback, supplements, etc. or did he just rest and get back to normal?

Good to hear that he is better. Keep him away from contact sports Just let him know that it could last twice as long next time. I know that is enough to convince me to stay away from activities that could cause another concussion.