My middle son was almost 8yo the last time he was at WDW 3 years ago. He refused to do RnR, and did ToT once (he'd also done that when he was younger--once.) He would do all the mountains, since they don't go upside down and there are no real steep drops.
His younger brother is 4yo and does anything that he is tall enough for. If he is apprehensive before going on a ride he is usually loving it when he is done and almost always wants to reride. he rode on ToT 5 times when I brought him (and only him) last February. Of course he is too small for RnR.
Last week we went to Hershey Park and they have a looping roller coaster that we could all go on. My 4yo was so excited to ride it, but my 10yo was almost in tears. We were all going, though, and he didn't want to sit out while we all rode, so he bit the bullet and went. Of course he loved it, rode it lots more times, and is now begging me to bring him back to WDW so he can ride ToT. Little does he know that if all goes well I am planning on bringing only him in January.
Maybe you can find some less intense rides at local amusement parks that can ease your son into the WDW rides. Before going to Hershey Park I really didn't think that my son would ever go on RnR, but now I really think that he will try it. And he'll have to go back on ToT since I'm obsessed with that ride!
T&B