You said that he is just considering it, and that he was going to go for a ride to see, how did he feel? You said he knows your not "happy" with him riding, but it's his decision. Believe that he will make the right one. Encourage him to volunteer, or maybe there is a shorter ride, you said he choose the quad-century option. He can always just ride for fun, or drop out if he finds it was over his head. What ever you do support him, do not go behind his back, or lie, that is horrible. It might be the greatest ride of his life- I hurt the whole way, I bonked on every hill, I was so hungry, It was so fun!