Get well and wishing your son and wife a full recovery. It is hard not to feel responsible, but one of the things about learning to ride solo or riding with other experienced riders, there are many things to watch out for that the experienced rider takes for granted. Off camber turns and crests with road fallaway sharply right or left when banking is something best learned solo and not at speeds approaching 50mph for 3rd gear turns, too much mass and momentum to carry through unknown twisty roads...
This is the time to evaluate what you did right and what you did wrong, and learn what strategy to use to prevent this from happening again. Quite honestly an experienced rider must make special effort to remember when leading less experienced riders, to tap the brakes and use hand signals if hazards on the road are encountered, but there are very few countermeasures for situation you and your son came across. Only your son and you can understand what needs to be done. Just another reminder to us all that whenever the riding season starts, we should practice, and do so often, with novices and entry riders even more so.