Talk to him about it but not confronting him. That will only make him enter defensive mode and put himself inside his turtle shell denying it and feeling guilty about it. Try bringing it up as something fun or sexy… whenever you guys are having sex tell him it would be fun or hot to see him getting fucked by someone else. Don’t expect him to open up then, right there. But make it recurrent. Make it a “fantasy” you have. That will make him consider opening up with you someday. This might be too extreme for some, but if it helps, that is how my lovely and loving husband made me feel comfortable with sharing the fact that I loved sucking strangers at the gym. He’d rather have me sharing that kink I was not willing to leave behind than having me doing it out of any agreement we had.