Well, that's a complex question; a lot of parents don't realize how little regulation of the camping industry there is. I'll check ACA guidelines (which camps don't necessarily have to follow--accreditation is optional, and camps don't need a 100% pass of the requirements), but camps can really do whatever they want regarding this.
At my camp, for many years, I had to write a note home to the parents for EVERY visit to the health center, including every time they came to get vaseline for chapped lips or anti-itch lotion for mosquito bites. I rebelled against this for two years and finally won in my last year at camp: if a child came for anti-itch lotion, I noted it ONCE and said "given when requested by child". It saved me HOURS of work each week.
For any other type of injury, including minor cuts and scrapes, I send a short note home. (It's actually a "Visits to the Health Center" form, with date, reason for visit, treatment.) As for calling home, I don't do it often, but then we don't have remotely serious injuries very often. If I think the child might have an "interesting" story when she gets home ("My tentmate kicked me in the head and my ear started to bleed!" when actually, it was an accidental thing and a tiny cut near the ear bled a bit), then I call to explain what happened. If a child has been sick / injured enough to miss several camp activities, I would probably call. Obviously, any visit to the doctor, I would call. If I'm sending a child home with a lingering illness, I call ("Just want to let you know that Maria's been feeling sick today, I gave her some Tylenol, but she'll probably be pretty miserable when she gets home" kind of thing).
Not having seen your son's injury, but based on what you say, it sounds like a case where I might not call the parents, but would send a longer note home explaining the circumstances and why my assessment didn't make me think it needed stitches.
Next summer, I would write a note on your son's health form saying that you would like to be told about all visits to the health center. I always comply with such requests. In my opinion, all camps should follow the protocol I've outlined here at a minimum, but I know they don't.
ETA: the only information I could find in the ACA standards says that parents must give permission for treatment (you generally give blanket permission on the health form; some parents specify certain medications, or that they want to be called first before anything is done), and that the camp must keep records of treatments given--also, that parents must be informed in which cases they will be notified about injury/illness. It doesn't specify what cases those are.