K..
You seem like a great mom, but I think you are worrying too much, honestly. It's just a scratch. It will heal & go away soon. It's not going to get infected & it's not going to scar.
I have a 5.5 year old girl who was/is very accident prone, and I have never treated a scratch in any special way, other than leaving it alone. I actually have never heard of people doing anything to a normal scratch, to be honest.
Can't you put a band aid over it? Wouldn't that be the best way to protect it & prevent him from scratching it? Just a thought.