Wow - what a great mom you are! You know it's important for him to do things like this, and want to let him go, even though you're scared to death that something could happen to him. (I am NOT a great mom - there'd be no way in hell I'd let my son go unless I went with him. But I figure my paranoia and overprotectiveness will give him lots to talk to his therapist about when he's older.)
Anyway, you should definitely talk with the friend's parents, as everyone has suggested, to help you feel better about it. You can even let them know that you're worried and will probably be calling a couple times to make sure everything is okay. Then pray, ask the Lord to watch over him while you can't, and have faith that He will.
As long as your son - (and make him and the friend's parents PROMISE)is wearing a life jacket, he should be just fine on the lake. This adventure will be good for him and for you.
I always remind myself of what I was like at that (whichever) age, and remember whether I was ready for something like that then. And in this instance, I would think, yep, I went camping with my grandparents and cousin at that age, and my grandparents never supervised my cousin and me (she was the same age as I) ....we'd go wherever we wanted all day, hiking, swimming, the rec hall on the campgrounds, and we were fine. We knew what we should and should not attempt, and we handled ourselves well. And oh, what great memories!
Best of luck making your decision. I will say a prayer tonight for the angels to be watching over your son, also. Blessings, J.