As much as Rogue One is my favourite Star Wars film, and as much as I love the space battle at the end and all the callouts to New Hope they got in there (all the while staying nice and consistent with New Hope's events), the one thing that really annoys me is Red Five blowing up. In New Hope, the call-sign is clearly associated with the fighter - they all have markings for their number - and it was perfectly reasonable that Luke got Red Five because they had more fighters than pilots. This was a real problem during the Battle of Britain and is one of the reasons modern military squadrons often have more pilots than aircraft. It's much easier to fix a broken plane than to wait for injuries to heal, after all, and it's also very possible for a wounded pilot to make it safely home and then die. But all that went out the window when Red Five blew up in R1.
Anyway, rant aside: remember that the markings on the New Hope / Rogue One helmets aren't "rebel" markings, they're the symbology of whatever military those squadrons belonged to before they became Rebels. As early as Return of the Jedi there was an effort to show a wide variety of kit and uniforms in the Rebel Fleet, to tell us that these people came from a wide range of backgrounds and forces, and Rouge One and Rebels built on that. (In my personal head canon, most systems had their own militaries during and just after the Clone Wars and as the Empire slowly clamped down on these local forces, some of them joined the Rebellion. I've always assumed that the forces we see in New Hope were ex-Alderaanian troops. The Kenobi series reinforces that for the ground troops at least.) After the squadrons left whatever military they were part of, they may have decided not to preserve the old traditions. Or they just may not have had the time and resources to do so when they spend the next two years on the run. Or maybe only a particular person or position could award new icons, and no-one with the qualifications joined the Rebellion. Or, heck, maybe the massive casualties in the Death Star attack meant that Wedge and whoever was flying that Y-Wing were the only folks left who even knew what those markings meant!
In any case, I'm personally 100% confident that the markings on Luke's helmet were earned by whoever had that brain bucket before him.
A lot of the markings on the helmets are probably just personal decoration, but the circle with the lines through it appears on a lot of helmets and maybe even on a couple of the X-Wings. (I'm not certain that's canon, though, it might just be secondary art.) Most unofficial sources assume that circle is some sort of squadron marking or national roundel. The model makers for New Hope put a lot of little details on the X-Wings, with many of them showing traces of different livery that got painted over, so likely the circle-and-lines is related to whatever military group the painted over yellow-and-blue livery represented.
As for your son's helmet: unless you're trying to recreate an on-screen helmet, I'd say embrace this as an opportunity to be creative. Heck, you could even use the helmet as a way to memorialise your son's achievements, adding "victory" symbols when he accomplishes something significant like good marks on a test or project, finishing a hobby project, attending an event in-character, or whatever other behaviours you want to encourage!
