Can I get my stuff back from a follower after he has left my service?

Solution 1:

The cleanest way to get your equipment back is to complete the current quest. Nobody dies, gets a bounty or tries to kill you. You're in a very unusual situation, possibly the only such situation in the game where an NPC who could be a follower temporarily has the follower tag removed as part of a quest.

Solution 2:

• You could kill him and then take the stuff, obviously losing him in the process and possibly gaining a bounty.
• You could pickpocket him, sell it to then buy it back from a fence to launder it.
• You could finish the college quest/line and then ask him to be a follower again.
• If PC version you could use console to do it (multiple ways), but this would be cheating.

Solution 3:

I had a similar problem. I had Illia as a follower but when I accepted a quest from the companions, she left with all my stuff. I tried to pickpocket her but got caught and had to kill her. Still, I got my stuff back and somehow never got a bounty.

So, to answer the question, if you don't mind killing them, that's one way to get your stuff back!