How low will the risk of testicular cancer drop from stopping masturbation?

It won't: Your risk of testicular cancer is unrelated to your enjoying your body. Further, you will probably not be able to stop the practice -- try it, and it's likely you'll simply end up unable to get to sleep. Do yourself a favor and ask your physican for a reliable information on sexual health and your special concerns. Being a man is difficult and you deserve a sound guide.