I had my jaw wired shut for four months once. Three things concern me.

Is he on pain pills? They usually go with the territory. Probably shouldn't do an endurance event full of narcotics. He won't be able to hear the body tell him he's in trouble. Bad thing in the heat. That may also be clouding his judgement- I was full of Demerol and thought I was Superwoman...

Is he rushing back onto the horse that threw him? What are his reasons for doing this even though he knows it's probably a bad idea? it's hard to come to grips with injury and missing events, especially if it's the first time you get hurt like that.

Baby food doesn't get through a wired jaw. I spent all day putting food in a blender and eating ice cream floats and could barely get the calories I needed- I dropped too much weight, and my sports activity was really restricted from the beginning. I doubt he can get the calories and electrolyte he'll need- this may be good, as he'd bonk and be scraped up by the SAG wagon early. God, I was so hungry all the time.
Ensure and all may be great for runners on event day- but a guy who effectivly hasn't eaten in weeks is in a totally different situation. Living off that stuff is survival, not good nutrition. Been there, done it, it doesn't make a good total diet over time.

Having been there, I don't think it's a good idea. And his doctor will say absolutely not to do this. Even when the wires come out your jaw muscles are atrophied and you don't go right back to eating normally. Soup with small stuff in it. He has a while to go.
Perhaps you need to play dirty pool for his own good. Get a freind to tell the event organizers his jaw is broken and wired- they won't let him play. Heck, tell him straight out that if he shows up like that he won't be allowed to start. And it is the truth. No matter how many Euro team jerseys he has, he's not Tyler Hamilton. There are liability concerns here for the event. Especially if he's full of pain pills. And another fall could be truly disastrous. he needs to consider that he could be a hazard to other riders.
He may sulk about it, but it ain't double knee and foot surgury. All athletes have injury and miss events, even whole seasons. I've been out for three years being put back together.
Maybe you all could go and help SAG the event? Just being there may be what he needs. Bonding with the other Citizens of the Bicycle Nation and all that.
Hope this helps

missliz