also with a little work you can easily change all normal open chords to 2-string chords and teach im in that way, so he can build on it very well later.
For example many players start with the G major not:
3 2 0 0 0 3
but
x x 0 0 0 3
Just make sure he is playing the chords with the same finger he is jusing in the real chords later so he jsut has to add the other fingers.
Then maybe teach him some very easy 2-chord children songs or just some little melodys with not more then 5 different notes, maybe just on the high e string.
By the way, I don't think that starting with Open D tuning will be a lot easier.
Hope that helps! :)