If you can’t get Slinky Dog Dash as a
DAS Advance Selection, you will be able to make a DAS Return Time for it in the park on your day of visit.
If you are really not sure about him riding it, you
might be better off getting a DAS Return Time for it.
DAS Advance Selections have a 1 hour window of time to use after the start time for entering the attraction. You may not be given many (or even any) choices of the start time. Although some people have reported using it after the 1 hour window had expired, there is no guarantee you would be able to. Officially, they expire and disappear if not used by the end of the window.
DAS Return Times are based on the current Standby wait time. They remain valid until used, cancelled/modified or until the attraction or park closes for the day. So, you can return to the attraction any time after the return time - if he needs time to work himself up to getting into the line, you would have a lot more flexibility.
If you are going to MK before you go to Hollywood Studio, Goofy’s Barnstormer is a good ‘starter’ roller coaster for people who haven’t been on one. It’s still thrilling, but it‘s only about 1 minute long ride. SDD is 3 minutes.
Other differences: Barnstormer is somewhat bumpy, while SDD is a smoother ride.
SDD has loud noises and lights, which Barnstormer doesn’t have.
Good luck and hope he likes it.