I know how your son feels mate. around the same age i used to clap, hit myself to able to speak a few words. I would be honest with him and tell him that just like anything, he has to practice to be able to improve. He has to discipline himself in order for his speech to get better. I read pages of book out loud in front of the mirror every day. I let go of all bad habits (hitting myself, clapping, moving my body or lips in any way shape or form) Thing is your body has to forget about all of those bad habits at some point and until they forget about them by him not doing them his speech will not improve. Sending much love to your son since i see myself in the kid you're describing.