Everyone reacts differently, so it's hard to say, but it's probably best to have someone go with him for support.
I met a man through our local support group I went to, who had his Gamma Knife early in the morning and then walked out of the hospital and met a friend for lunch. He was back to work in 2 days and doing great. I went to the same Gamma Center, early in the morning, but did not have my treatment until mid afternoon. I had a massive headache immediately after the treatment and was given an IV injection to take away the pain and the nausea. It knocked me out for about an hour and when I woke up I was pain free and had no nausea. I did need someone to drive me home and once there I had something to eat and then went to bed to sleep off the drugs I had been given. The next 3 days were rough because my eyes swelled up in a reaction to the anesthetic from the head frame being applied. I was basically at home using ice packs for 3 days before I could get the swelling down and the bruising cleared up. Other than the swollen eyes I felt fine, no headaches or problems.
Best of luck!
Cathie