I'm so sorry to hear that! I recently fractured my spine in a horseback riding accident and I also have AS. The trauma surgeon said that it would be hard to say how it will affect the AS because everybodies AS is different. One thing that I did do and seemed to help was getting into physical therapy in a warn water pool. Additionally I would chew a couple of TUMS every day to increase my calcium intake. It's important after a break to eat a high protein high calcium diet to heal quickly. It may take longer to heal because of the AS or may heal faster it's a surprise. I also with drs ok stopped taking the prednisone I was on since it leeches calcium from your bones and makes your bones more succeptable to fractures.
The best advice I can give is for him to listen to his own body and it's limitations and keep in contact with the rheumotologist. I wish him a speedy recovery!
FYI I am not a medical dr, I have been diagnosed with AS for 6yrs (I am 26yrs old).