I agree that it's important to get him going on playing simple chords and substituting some open strings for the barre chords, etc. You want to get those things in his playing early on, although some open tunings can be fun.

DADGAD is a good tuning that is instantly rewarding. The tuning itself spells out a Dsus4 chord, so all the chords have a particular sound. However, you can play single notes on the low D string and let the open strings ring and it sounds great. The tuning is :

E down to D
A
D
G
B down to A
E down to D

Let me know if you have any questions about this.