If you're thinking to pursue this for a good energy drain, you'll eventually want to train him to pull - they work harder and use up that energy more quickly that way!
I would wait until at least 1 year old for "real" running, and definitely no pulling before that. Sand is harder work to walk in, just like it is for us - so even if you're only going 1/2 mile, it will be roughly equivalent to 1 mile. But starting smaller distances is a good plan (especially on lower impact surfaces), and this is also a great time to work on control! I'd also make sure you're really comfortable using and maneuvering in the skates before you bring Dean with you.
Before you get going on wheels, take time to train those control commands first! (They're eminently useful.)
I bikejor with Zage, and these are mostly what we use:
Hike = go
Hike up = pick up speed
Easy = slow down
Whoa = stop
Gee = right
Haw = left
On by = pass that(ignore that)
Straight ahead = no turn (at an "intersection")
Gee(haw) over = go to the right(left) side of the trail
Line out = go out (until you feel tension on the harness)
As far as harnesses go, there are a number of options/styles, even within the pulling world. I use an X-back (custom fitted for Zage), and see many many other shepherds in X-backs. I see other styles as well, but can only speak from experience for the X-back. If you decide you only want him to run next to you, I might skip the harness - because no matter the style of harness, they are meant to encourage pulling. And if he has any kind of pull drive, he will!
Dover von Ziegeln Zage (GSD, 3 yrs)
Dutchess Lena of Black Lake RATI (Keeshond, 12 yrs)
“Once you have had a wonderful dog, a life without one, is a life diminished.”