@Fristepha: I had a spiral fibula fracture in August 2012. It's now April 2014. I was playing roller derby at the time. I was 'off wheels' for three months and one week, however I did not return to full contact play for another two months after that.
I would say it is absolutely imperative that your son takes plenty of time to heal fully, including getting plenty of physiotherapy, and committing to doing whatever exercises he is told to, every single day. The ligament damage caused by the forces which lead to a spiral fracture is actually 'worse' as in, ligaments take much much longer to heal than the bone does. So, while there is probably no reason why your son can't get back to football, if he jumps back in too soon he will likely end up with longer term problems - if he unconsciously favours the damaged ligaments, his joints will slowly start to compensate, so the knee gradually loses a little alignment and that can lead to more problems down the road.
So, tell him to take his recovery seriously, just as any athlete would. Best wishes for his healing and tell him to hang in there and follow the physio's advice!