Well, first chance he got, he took it.
Junior Grognard was keen as mustard to get a session in and last Sunday we did do a bit of gaming with him as DM and me as player.
JG drew a map on some A4 sheets, no dimensions as such and he laid it all out in one go rather than piece by piece but it all went very well. He'd remembered some of the features of the training dungeon and most of the combat sequence, which was good. We had to go through a couple of character roll-ups so that he would get the feel for what to use, the names of the abilities, dice to roll for money. I'd got my PHB out but I also had OSRIC printed out (without the monsters) and he may run from that instead.
I took a thief and a cleric down and both got killed several times (he's using the 'get killed and go back to the start' idea from the Training Dungeon). We were mostly facing kobolds who got some suspiciously good hit rolls and as the thief only had 2hp, he became intimately acquainted with the floor very quickly. The cleric, on the other hand managed to wangle scale mail and shield plus 8hp and was able to finish off several kobolds before an encounter with about eight of them plus bugbear sealed his fate.
JG's enthusiasm and relish in laying it out and bringing it on carried him through - he didn't have access to a quick reference for the monsters (THAC0, HD, AC, damage etc) so I've knocked something up and printed it off as a kind of ready reference sheet, very like the monster summary at the back of the DMG. Basic humanoids, low grade undead and creepy crawlies should see him through the first sessions.
I'm also doing a quick start primer for 1e for any of his chums he manages to recruit. It's heavily adapted from Chgowiz's S&W Quick Start but of course the terminology needs to be altered, plus some of the stats. There is still a lot of work to do on it; I need to let JG read it and see if it makes sense to his age group. It needs to be exciting and enticing enough for seven year olds.
So in summary - JG did a great job and I think that as he does more sessions and learns to build on what works and winnow out what doesn't and gets a feel for what monster levels work with a party, he'll get better and better. He needs to talk the game up with his friends (or at least offer them free pizza - thanks, ZeBulette!) but that's probably going to come as he gets more into the DMing side of things.
As he went to bed that night, I wished a welcome to 'the newest DM in the world'. It can be the toughest job in the game but also the most rewarding - I'm sure he'll find that out.