Oh, goodness gracious. Your son is 22 years old and intelligent and makes good decisions. He will be fine. He has the internet to research EVERYTHING before he goes, and will be much more well-informed than if you or I had wanted to do something similar when we were his age.
One of the summers when my H was in college, he worked for a boy's camp in Colorado, as a counselor. He took a group of young teenagers backpacking into the Rocky Mountains for days. They did a number of 14ers, and everyone survived. He has such fond memories of that time.
Encourage your son to do this and give him your blessing. Ask him how he'd feel about having regular check-ins with you (maybe he'll need to get a satellite phone) so you know he's safe... like every other day or so.
Two years ago, two Stanford University students decided to hike the Appalachian Trail, Continental Divide Trail, and Pacific Crest Trail, all in one year, which very, very few people have ever done. I followed their progress on Instagram, and was so impressed with these two young men. Again, with the internet, it's so easy to be that much more prepared these days.