Ask him questions like:
What does a polyamerous relationship look like to you?
How do you show your partners respect in your relationships? What does it look like?
What is your motivation for polyamery?
How do you manage your time when it comes to time spent with partners? Do you realistically have the time and, enough of you, to add another partner to your life?
What kind of person is your current partner?
Have you ever experienced arguments with partners/potential partners regarding polymory and, how did you both resolve them?
What does empathy mean to you? Can you tell me about a time where you had to empathise with the feelings of a partner? Perhaps to reach a compromise?
How do you handle times of jealousy if it occurs?
I do believe that successful polyamerous relationships are possible. But you need to be more epathetic than most (and excellent communicators). All parties involved do. Because its not just 2 peoples feelings to consider.
I suspect unfortunately, many men just say they are polyamerous, when infact they are just shagging about and treating all the women like shit.