I had my shunt placed before knowing the type of tumors they were dealing with. My doctor expressed the same concerns in that tumor cells could more easily spread. However, I was suffering with such severe hydracephalus that he was more concerned with the possibility of siezures that could be fatal. So, we decided to go ahead with the shunt. One of the big things to consider is the age of your son. If he still has a lot of growing to do, they will have to go in periodically to extend the tube to ensure it's working properly.
All that said, and even though my surgery was very disruptive to the core muscles in my stomach and chest, the end result was a much easier time in life. Before they put the shunt in, I was suffering from severe headaches sometimes 4 or 5 times a day. Once the shunt was in place, the headaches stopped. It was an instant change. If you have any other questions, let me know. If I can answer them, I'd be more than happy to!