It sounds like you are open to trying this. There are not really any formal rules to polyamory. The closest is something based on loving many people with full knowledge and consent of all involved. The biggest emphasis is on communication. It is ok to feel jealous, but we also try to use it to better understand what that jealousy represents like insecurity, fear or envy.
People can cheat in polyamorous relationships, but my guess is it is a lot l3ess common than in monogamous relationships because some of the reasons tyo cheat have been removed.
Make sure you are very clear with each other. for example, if you say you are comfortable with him seeing her, but no sex, that is too vague. Some people say that a blowjob is not sex. Some think that getting to second base is. So make sure you are on the same page.
If you are open to the idea, then why not try it? If anything it is a good life story no matter how it turns out.