I remember reading years ago a message posted by a parent of four children. He stated that with his four children, he noticed they progressed through stages with rides. A ride they may like at age 4 (POC & HM), they may dislike when they were a few years older. I have since been able to observe the same in my own children. My son was fine with Splash Mtn., POC and HM at 3. At 6, he was fearful. We've already had him in the preshow at HM, then he would opt out prior to boarding the Doom Buggies. We've conjoled him onto POC, but we do it very carefully. We try to respect our children's fears and concerns and deal with them honestly.
BTW, last trip, son was 6 and fearful of POC and HM. But tried and *loved* Space Mountain! Go figure!
If your son doesn't want to go and you have another adult with you, you can always parent swap in order to see and enjoy the ride for yourself.
Good luck with whatever you decide to do!