There is a family of similar tunings "near" to DADGAD, including Open D (DADF#AD), Open Dminor and (I think) an open G.

Would have to look these up once away from my desk, but the point is if the fingerings are possible he could get quite a lot of variety in sounds by only retuning one or two strings away from DADGAD.

Hope this helps.
Best regards
John Robinson
http://www.JulieEllison.co.uk