We have a scaredy cat, too. DS5 may be the only little boy on the planet who LOVES Captain Hook, but HATES Peter Pan's Flight!

He just doesn't like dark places where he can't see where he is going. I say start with Dumbo and Aladdin's Magic Carpets, the Railroad, the Swiss Family Treehouse (what 5 year-old boy wouldn't love a treehouse like THAT?!

) and the Riverboat, then move up to the Jungle Cruise, Tea Cups and Tomorrowland Indy Speedway. I would DEFINITELY avoid: Snow White and Stitch!
I agree with a PP that said start him out in open rides (notice the theme above). I highly recommend Bob Sehlinger's "Unofficial Guide to WDW." I'm sure you can check one out of your local library. There is a great section on "Disney, Kids and Scary Stuff" that's been largely the same for the last umpteen editions, so an older version should be good if that's what you are reading the book for. Me, I buy and read every new edition, but I'm a geek.
One other suggestion is to get him some foam earplugs (yes, here I go again on the earplugs

). We bought some foam earplugs on a string (love the string--makes'em easier to keep up with) for DS and used them EXTENSIVELY on our two trips to Disney last year. The foam ones you just roll between your fingers to squish, then insert them gently into the ear (can't get too far--don't worry), and hold them about 5 seconds while they decompress. DS just really did much better when he only had one sense overstimulated. I don't know who decided that all attractions need to be as loud as a rock concert, but the WDW attractions are LOUD! We also used them for all the nighttime shows.
Good luck!