Sarah Jane ~
As you know, we're all unique and I can only offer you my personal experience. I not only drove myself to and from the 26 FSR sessions (60-mile round trip) but never felt any real fatigue. I was recently retired so I didn't have job stresses to deal with and no small children to care for, either, so that was a factor in my favor. Being blessed with a high metabolism rate and good health all of my life I've been able to avoid regimented exercise routines and can't comment on whether your workouts will be affected. My first thought is that you'll know soon enough once you re-start your normal exercise routine and if it's too difficult, you'll put it on hiatus. You can't put your son on hiatus but I doubt the FSR will be very debilitating so this shouldn't be an issue.
The point of FSR is 'zap' the tumor with relatively low 'doses' of radiation so as not to damage surrounding brain tissue and to reduce any adverse effects of the radiation. This worked perfectly for me. I received just over 1 Gy radiation with each session (27 Gy total) and, as I've stated, did not experience any negative effects. Again, my positive FSR experience, while encouraging (that's why I post it) is not a template or guarantee that every AN patient undergoing FSR will have the same experience, as other posts on this thread demonstrate. However, I'm optimistic that in your case, the FSR experience will be relatively trouble-free.
Jim