I think the way to go about it is just as you said. In other words, ask him to be honest even if some of the things he says/does might upset you. Ask him to challenge you. Challenging not in the sense of being confrontational with a client but pushing them towards self-growth or exploring different ways of doing things etc (rather than following your comfort zone)

You can stress that you will be able to take it and work on any upset/rupture with him because your goal is to get better, improve, whatever word applies.

A decent T would not purposely be mean to a client or tell them they are doing something stupid... but you can get an 'edgy' T that, while providing support, gently challenges you towards a direction of growth (whatever that means to you).