Not OP but Hatchet was an assigned book when I was in 5th grade (American Midwest school), it’s pretty common for early school age kids to read. It’s got some slightly intense scenes in it, but nothing that 5th grade me was too scarred by (the scene where he has to go into the lake to get something from the plane and sees the corpse of the dead pilot was pretty scary to 10-year-old me but that’s about as bad as it got.)
Hatchet was one of my favorite books growing up. I believe the first time I read it I was in 4th grade if that helps you gage (it was pretty common in 4th and 5th grades if I recall correctly). That said, my mother was a librarian and didn't care what I read as long as I was reading (including Faulkner and East of Eden when I was in 6th grade which was way too mature for that age :-)) so depending on your son maturity your milage may vary. That said, Hatchet is a great a book and I give it some credit to my life long love of the outdoors and adventure.
He’s 7.5yo. Don’t think he really understood The Secret Brian’s mom had, and probably still hard for him grasp severity of the situation, but he was very engaged and enjoyed the book. I don’t recall how old I was when i first read it… but I do recall accidentally finding a Gary Paulson book in the adult literature section of the library and quickly realizing it was not for a young juvenile :D
Don’t let your kids read Roald Dahls adult literature either, Switch Bitch is no Giant Peach
I found Hatchet when I was in grade 4, and damn near read it straight from the library checkout counter until I was done a few hours past midnight. I'm pretty sure that my parents made me take a break for dinner.