I took my then 7 and 3-1/2 years olds to E-night last October, and it was great. Of course, in off-season, E-night went from 7 p.m. to 10 p.m., so it wasn't too late for them. My 7 year old loved Buzz Light Year Space Ranger Spin, and we went on that repeatedly (walking right on) for nearly an hour. As long as the rides that are open for E-night are ones that your DS enjoys (there's a list somewhere) and if he can stay awake, I think it'd work out great!
We'll be there again in a couple weeks but will probably stick E-night as it will be from 10 p.m. - 1 a.m.; my kids are on early schedules and don't sleep late when they've stayed up late, so I'd ultimately pay the price for keeping them up that late. Lots of kids are either already on late schedules or will sleep late if they stay up late though, so it could work for some people.
Have fun!