When you join the fire service you dont automatically start driving, you work your way up to it, so it may not be totally crucial (like postdog said)
It is really hard to get into the service, and the DD shouldnt have any bearing on his ability to perform as a fire fighter, but if there is alot of competition he might have to apply a couple of times.
A friend of mine is in the service and he applied twice before he got in, sometimes they reject you just to see if your serious enough to apply again! Tell him not to be put off, every application is considered on merit, and the DD will not automatically strike him out x