When a group takes of in the front like that: you in fact have two groups that leave the starting point at the same time. I would point out that he needs to keep the required number of adult leaders with him and let him go. Then I would concerntrate on making sure the the second group was properly led. If I did not catch up by time to make camp, so be it. The second group will get more out of the trip, both in fun & skills.
If you force the SM to stay with the second group he would probally be a pain in the rear (no pun intended).
Having said that. The trip leader is responsible to the whole group & not doing what is fun for him.
