I vote for Proton all the way. It sounds like you are being positive and you should be as the odds are excellent on a great outcome. The best way I can explain is to tell my story. See below...
I was diagnosed with HPV 16+cancer of base of tongue March 2023. I am 58. I had TORS surgery to remove tumor and lymph nodes through right neck dissection. 2 of which had cancer. Radiation followed about 45 days later. I chose Proton vs Photon Radiation as I feel it is much less damaging . In my opinion Proton is more targeted with less side effects. However, It is a battle as insurance companies do not want to pay for Proton when Photon is 50% less costly or more. Proton is like a pencil beam hitting the target and stopping. Photon is more like a flashlight beam which travels through the target area. I had to drive an hour to Proton treatment. Alot of people drive much farther. I am currently 2 years post 30 rounds of Proton Radiation to "cleanup" any cancer that may have been left behind from HPV16 base of tongue and Few Lymph nodes . I opted out of any Chemo treatment. l was very worried about going on a feed tubing , Dysphagia and Not being about to swallow (eat or drink). I started doing mouth, Neck, Tongue and swallowing exercises prior to proton Radiation. I would look into a Speech therapist they can help with this. It was a struggle to eat during radiation and I FORCED myself. I did manage to stay off a feeding tube but I lost about 20 pounds. You will lose your appetite and everything has no taste. However as far as today. I can eat and drink about anything I want. My taste buds are about 95% back, My throat mucositis has almost gone away, Saliva production is about 95% back, My stamina is back to about 95% . I had a throat scope and CT scan every 3 months and now It is every 6 months. I also have a NavDx test done. You should also ask about the NavDx Blood test for HPV 16. It will become more important once he has completed his treatment to monitor any HPV 16 cancer activity in his system. I continue to be cancer free as of today. Trust in your faith. God speed!