Dear SD,
First of all I send you my very best wishes for your son's surgery. Kids are very resilient and before you know it he will be up and running!
I understand your worry about post surgery activities as my son had both surgeries last year at the age of 7 and he is a very active boy! I was very nervous about what he should do or not do. After surgeries he will probably need a little bit of time to recover and his own body will tell him that anyway but I have to tell you kids do bounce back very quickly and before I knew it he had all that energy again.
Our surgeon told us that it was very important to avoid any high contact and strenuous sports. I guess it takes about 1 year for new blood vessels to grow so it makes sense that head injuries or physical exertion should be avoided. However it doesnt' mean that your son should stop all activities.
What is important is to set boundaries and allow some type of activities or focus on new activities. Our boy was able to bike, swim, ride his scooter, play. We just avoided signing him up for soccer right after surgeries, forbid the skateboard park (he didn't like that part at all as he was getting pretty good at riding his bike on it   )). I also signed him up for painting lessons with his brother as they both really enjoy arts. Maybe there is a new activity you could find that your son might be very interested in and that doesn't involve you pulling your hair out of your head  .
What is also important is for you to learn how to relax and to know that you won't always be able to protect him. You slowly learn to let go of the fear and trust that your son will make the right choices. We sat down with our boy and discussed his condition and why it was important for him to be very careful so that he understood why there were some limits. It is not always easy but you need to stay firm and most of all trust your instincts.
Our boy had a stroke prior his surgeries which left some weakness in his left arm and hand so  activities were a big part of his recovery. It was also good for his spirit, knowing that he could be like all the other kids...
Six months after surgeries we started X-country skiing which was very good for balance and coordination. We are now 1 year post surgeries and I am able to allow more and more as long as proper safety equipment is worn at all times!!! 
You won't be able to tie your son down so I think it is best to let him regain a zest for life as soon as he feels up to it. You need to trust that everything will be ok!
All the best to your family!
Gazou