I would tell him. I have three children, ages 3, 5, & 8. They all know and understand my situation, and where the disease can lead. They now serve as my food police, prayer warriors, and general moral supporters. They also help me juice (to the extent that they can).
You need your loved ones to cover your back. Encourage your son to get involved in your healing. He can help you with juicing, etc. Also, you can teach him valuable lessons about health an nutrition. You can take him to the Farmer's market, etc. However, he needs to understand the reason for all of this. Not to mention that he probably already knows, and your lack of candor will only isolate you from your teenager.