Hmm, that's all but unheard of for him, a 500 at +6. cat(2)_steam
I wish I had more ideas for you - you've tried lowering the dose and the nadirs get quite a bit higher, and you can't raise much b/c you risk the steep drops and low nadirs, so there isn't much room there to play with the dose. The only options that make sense to me would be early shooting, which is a pretty daunting task, or another insulin.
On the plus side I do think he has a really good chance at continued progress and continued lowering of the dose, but I don't see how you get there from here on 12/12 PZI shots. Also on the plus side you know he can get good nadirs and you have a window on the dose, so an insulin switch wouldn't be too traumatic I wouldn't think. You should get to the dose pretty quickly, start low and build up and you should be at these levels very soon, and then hopefully he would benefit from a flatter curve. No guarantees, but that's what I think I would do in your shoes.