What I got when I graduated from high school and have been doing ever since is getting a nice graduation picture frame and putting a hundred dollar bill in it. Works for everybody.
I would imagine his parents are buying him a laptop for graduation, as everyone seems to these days. You might offer to buy a nice bag for it to go in.
$100 gift card to his favorite fast food place is an option: teenage boys eat a lot, and they often don’t get home much during senior summer.
Does he do any film photography or strictly digital? If he develops his own stuff, just go put yourself in the hands of the guy at the camera shop! That is a never-ending expensive hobby.
The best high school graduation gift I received was a nice tower fan with an air filter. Seems useless and dumb, but if he’s going to be living in a dorm, he’ll grow to love it.