It seems to me that there are other players involved here who need to step up.
If you want to make sure he has a chance to grow as a DM, and you want to avoid too much in the way of hard feelings, I would suggest to the group that *everyone* take some time in the big chair. Each person would be DM for one 'adventure' (which may be more than one session), and then the responsibility would shift.
The adventures need not (and I suggest should not) occur in the same game world, or even in the same game system. It exposes everyone to some different play styles and genres, gives each GM plenty of prep time, and gives some breathing room if someone isn't very good at it.
Eventually, people will likely develop favorite games, and some people will want to drop out of the DM rotation, so you'll likely eventually end up with 1 or 2 games, but in the meantime you've had a change of pace, and the less experienced DM has had time to season.