We went for a longer ride on Sunday and after lowering the seat he does pretty good. The very top of the seat post is tapered so I didn't drop it all the way in. I may try it and see if it holds, but he's doing good. I had my daughter in a trailer on the back. That was fun. The shifting was a little imprecise, but I think I'm getting it dialed in.

Anyway, I did have a slipping feeling that I asked about in the mechanics forum, if someone wants to take a look in case it's a tandem specific thing.