My wife and I are going through a similar situation. She's 26 and I'm 34. She recently came out to me as poly and it didn't go over so well. My advice is to explore the topic as a team instead of just informing him that this is something you want to do. Be patient and give him time to process his feelings. Make sure he knows you still love him and that there's nothing wrong with him. For me it helped a lot that my wife said things like "I'm being completely open and honest with you about my feelings" and "I'm talking to you before taking any actions". For me it hurt a lot to hear "These feelings started when you did ____" and "I'm doing this to bring home positive energy for you".
I had a ton of questions for her, like "Is it okay if I just pick up some chick and have a one night stand?" Answer those questions honestly and have your own boundaries. At one point, I asked "Is it okay for me to fuck a prostitute?" Since she had no boundaries for me, I just thought she was desperate to be with the other guy at any cost.
That's my experience anyway. You can read more about it in "Help... my wife is moving too fast."