Telling him that you hope he will wait to engage in such activities is honest on your part, but naive. And more importantly does not invite him to share with you, or come to you for help if he needs it. Demonstrate your availability to talk through any questions he may have but don't be pushy. Let him know that if he does experiment with alcohol that you are available to him if he was in a situation that spelled any danger, including driving or being driven by someone who was drinking. And let's face it.
Mistakes can happen during exploration.
Contracting like this does not change any of the values you have taught to your child. Nor does it change the fact that you would prefer he wait to explore these areas. It does however show him that your head is not in the sand, and that your relationship with him is durable and reflects his reality. Teenagers are not adults nor are they children. As parents, we are also beginners at this period of our children's development.