I would think this would be linked the involuntary movement that his muscle are doing rather than a conscience thing he is deciding to do. You may be able to talk to the doctor about ways to keep his hands out of his mouth, but I would think the only way would be to restrain him and that is not optimal. I know there are so many treatments and even new test treatments they are doing on patients with cp and maybe there is a study out there he could get involved with directly related to the muscle function, which would probably be something you would wan to consult your local hospital, they might be able to help with that. Good luck to you.