Never said grizzlies aren't dangerous. But statistics show that the black bears the OP and his son intend to hunt are more dangerous. They are more likely to stalk and kill you. Rarely will grizzlies do that. They are more dangerous in surprise confrontations. And grizzlies are certainly much less dangerous than driving to the grocery store ... or the cemetery. In fact, this afternoon I was nearly creamed coming back from the grocery store when some nut blew a red light at the intersection crossing Trans Canada freeway. I'm much safer living with grizzlies than a city full of humans!
For over five months every day I worked in Alaska around scores of brown/grizzly bears. Sometimes within six feet of them. On average probably no more than fifty yards. I had only five incidents that I can recall offhand. Never deployed my bear spray. Those were pretty much all "habituated" bears ... which I think are the most dangerous. Too unpredictable. Real wild grizzlies do not cause me anxiety ... if the encounter is mutually without surprise. Incidents of wild grizzlies attacking more than one person together are pretty much unheard of. Bluff charges are not unheard of but a grizzly actually tearing into a group of people ... no.
I'm in pretty good shape for my age but I'm not playing basketball with fifteen year-olds. They'll kick my arse. I'm sure they could also handle the hills of central Idaho better than most of us.