I would ride your GS and get a GS for your son as well. IMO and I've done a lot of it; riding a 650 single cylinder like the DR or XR any distance is brutal. About 20 years ago I had as my only bike an XR650L and I rode it all over N.America. Even then as a 19 year old it killed me. I could ride maybe 45 minutes before I absolutely had to have a break. I have an XR650L still today and rarely ride it because it's just so uncomfortable. I've also owned a DR350 that I felt the same way about.
I think the GS is more than reliable enough for your trip.