I'm going to agree with everyone else and say that you should let him experience his relationships himself, but I'm going to give you an example.
A few years ago, we invited a friend to have a threesome with the female members of our quad, as long as his girlfriend was okay with it. We gave him a couple weeks time to get back with us after checking in with her. He came back saying she was not only okay with it, but wanted a relationship with my boyfriend, Wendigo. Without checking in with her, we went ahead with it, because this was a life long friend of over 15 years who we trusted implicitly. A week later, I bumped into her at the doctor's office and she said that she hadn't realized Wendigo and I were sexually/ romantically involved when her boyfriend asked her. She thought we were "cuddle buddies." She wasn't interested in being sexual with Wendigo, but was interested in a cuddle date. I was okay with that, thinking she'd discussed this with her boyfriend. I let it alone. But talking with Wendigo, I realized our mutual friend was still talking up a sexual/ romantic relationship to him. I started to worry that he was going to get hurt. I knew that he wouldn't pressure her for more if she told him she didn't want that, but I knew he was crushing on her and hoped to save him from pain and embarrassment. They had their cuddle date and Wendigo gave her a massage, but she kept insisting his hands stay above her waist. When her boyfriend came home, he implied to Wendigo that he should just give it time to warm up to the idea. Wendigo expressed his frustrations to me and I tried to tell him what I knew, but he just saw it as jealousy at the time and was upset that I'd talked to her about us at all w/o asking him first (we weren't out to everyone back then, but she asked and her boyfriend knew). It was probably 3 more weeks of our mutual friend telling him one thing and his girlfriend telling him something else that started to take the blinders off his eyes. And then I bumped into her at the doctor's and she told me she was still thinking about the threesome offer and it came out that she'd told her boyfriend she wanted to know if the threesome happened. She told me she'd just found out she was pregnant and her boyfriend had proposed and I panicked, the threesome had already happened. I went home and contacted Wendigo and shared what she'd told me, and showed him her chat logs with me where she was telling me that she didn't want anything more than friendship with him and that she saw sex as a spiritual connection that could only be shared with her soul mate. I showed him the strange messages her boyfriend sent me when I confronted him, saying if I was jealous, we could call it all off. And he showed me the lies her boyfriend had been feeding him. Wendigo realized that we were all complicit in him cheating on her AND that he'd used both his wife and girlfriend to do so. He was beyond furious and his wife decided she could no longer be poly or apart of our quad. In the end, Wendigo realized I hadn't been jealous, but he had to get hurt in the process to see that. Because even when you have first hand knowledge and the best intentions, when the person you love is interested in someone else, you're only going to come off as jealous until they see or hear it with their own eyes and ears.