DAA vs ADA

I'm was a trumpet player years ago and know nothing about stringed instruments, but my son is learning to play.  While watching him strum I see that he often misses the fat D string furthest away from him.  Could we swap the D string with the middle A string to get a fuller sound?  Would it work?  Would we be beaten about the head with a possum board by other dulcimer players if we did that?  He is playing noter drone just using his finger on the melody strings. 

I've tried to help him but and I cant even use a pick properly and just use strum over the strings with my index finger so I don't want to teach him my bad habits.