My heart goes out to your kid to have four surgeries so far and still has to have further treatment!
I am not a physician and cannot give medical advice, but in my opinion, I would suggest getting a second opinion for your kid.
My understanding (and I could be wrong, and I also have not seen your kid's MRI nor understand his full NF2 history), is the reason most acoustic neuroma specialists use Cyberknife or Gamma Knife instead of Proton Beam is that there is a better track record of acoustic neuromas being successfully treated with Cyberknife or Gamma Knife than using Proton Beam. Proton Beam treatment is wonderful for treating many kinds of tumors, but its success rate with acoustic neuromas has not been the best. If that's true, do you want to go with a more expensive option which does not work as well?
With that said, I will repeat that it could be that for your kid's specific case, the Proton Beam treatment would be best, but I would recommend that you get a second opinion from a specialist who does not do Proton Beam therapy just to have another perspective.
I wish you and your kid the very best for health and well-being!