Both of my boys rode it when they were 4. However, we did prep them ahead of time!!! If time allows, you may want to ride it without him first.
I know you said you watched a youtube video, but it really is scarier in person (but not "scary").
My recommendations, keep it fun by "looking for things".... distraction can go a long way at preventing being afraid!!!
Don't stare or point out the portrait that turns into a skeleton! Focus on finding the room with the funny paintings. In that room, try playing a quiet guessing game of "can you remember what's the below the lady with the parasol?" "What color are her slippers?"
Remind him that it's going to become dark, so let's hold tight to wait for the scream & lightning. Those tell you to start counting because when you reach 10, the door opens to go into the ride. (we didn't look up when our DSs were young).
During the ride, I think the knocking and banging on the doors is quite creepy!!! Seems like monsters in the closets to me. We focused on looking for the clock, then the black bird with red eyes, then the sign with "home sweet home" (or is it tomb sweet tomb), then look for the skinny shaking dog... you get the idea.
Those pop up "heads" are scary, too!!! But the singing ones, and the opera singer are kind of cool. During the attic scene, we looked for luggage and chairs & didn't pay too much attention to the scary woman!!!
You can also remind your DS that you see the FUNNIEST pretend ghosts at the end and that you've heard that they sometimes you even get to tickle them (if they appear in your ride car)!!!
Anyway, destraction always worked for us and this is still one of our favorite rides and we still love looking at the "background" parts of the attraction.