NCAA rules permit a win in less than 5 innings if the coaches agree before first pitch that it's a "staff game" and no one will be going long.
NFHS has no parallel rule. The 4-inning standard is always in effect.
In fact, NFHS doesn't even provide an exception for slaughter games, but I know many a scorekeeper who would fudge it for a 5-inning blowout.
Your son therefore does get the win as jfsbndr states.
I don't know how you could get a 4-inning save. If you come in with the lead already in place, then if the pitcher went 4, the game should end in 7. If you come in and hold the lead for more than 3, the starter must not have gone all 4, so it would seem that you would always get the win in that situation. And if you get the W you cannot also get a save.