I liked HSM a lot and thought it was fun to ride. I think it's dark, but not dark enough that I can't usually see the track ahead of me (especially if I'm in the front row). One thing to note...if you think your son might really like it and would want to ride more than once that day, I would consider getting him a FP in the morning. HSM FPs are probably the most popular FP at this point or at least as popular as RSR, so they do/will run out. If you both get the FP, then you have the option of riding it with him (and seeing the limited theme-stuff added to the queue - not much really). If you choose not to ride it, then he can use your FP and ride it twice during the FP window. It does mean you won't be able to get another FP for 2 hours or until the time window opens, whichever comes first (except WoC and Buzz), but if he likes the ride, it's worth considering because the standby line can really build up quickly.
(Another note, if you go to DL on Saturday morning, keep in mind that it is the EMH/MM day, so folks with EMH/MM will already be in Tomorrowland when the park opens to the general public. HSM is open during EMH/MM, so there will probably already be a fairly considerable line by the time the park opens. However, they do not issue FPs during EHM/MM.)