Anxiety...

There's a kind of anxiety I've been noticing in Jack lately, and today it became something we just couldn't ignore.
We might be lighting out for a walk up The Hill, or we might be hitting the trail, as we were today, at Redwood Regional park. Suddenly, Jack will balk at going any further, like he thinks there's a ghost right up ahead.
If he's off leash, he'll actually turn around and head back.  If I call him, he'll wait for me.  If I pursue, he'll just stay ahead.  I have to stop, and demand his recall with a treat.
That's what happened today at Redwood.  I called him back, and we proceeded on leash.  He was fine for a little while, but soon turned into a pull toy.  I was almost dragging him down the trail.
I thought he'd be fine once we got down the trail a bit, but my own anxiety kicked in: what if he's having another crisis?
So, we turned around and headed back, Jack now leading the way.  Off leash, he was fine, waiting for me when I asked.  He just laid down with his head on his paws when we got to the car.  There was no trembling, or any other symptoms I could find.  He was fine when we got home, smilin' and waggin'.  I gave him a quarter of a prednisone tab(1.25mg), and he's been chillin' on the couch this afternoon while I stripped wallpaper.  Nothing wrong with his appetite!
Dang, I miss our great hikes together.  I think I need another consultation with the Vet.
1) Anxiety?
2) His 'aging'.. are there any supplements he's missing?
3) My buying DOCP, and giving the shots at home.
4) "Cumulative effect" of DOCP?