How to tell a date I'm poly and married?

I am married for 9 years and understood myself as poly about 5 years ago. My husband was pretty open to the idea then we decided to be open to other people. All our experiences with other people so far were sexual but now I'm having a big crush on a younger guy. I asked him out and we have a date later this week.

Thing is, I have no idea how to tell him about my husband, specially because he is younger than me (I guess he is ~23 and I'm 33) and I have no idea how emotionally mature he is. Any tips?

P.S. My husband knows about this guy and is pretty cool about it.

P.S.2 my husband is straight and would not be involved with him. This would be only between me and the guy.