You might ask your university's research office to check with the sponsor of the grant funding the contract. If it's a Federal grant, there are likely guidelines concerning openness and transparency regarding collaborators or co-PIs that you might be able to use to open things up a bit. If the former supervisor has to operate under these conditions with the Feds, then most contracts funded by Federal monies have to follow the same conditions.
Great suggestion. We work in different countries but I think this could still help me articulate a more balanced perspective. Thanks!
Other thoughts also welcome...he's a bit of a tough negotiator!