I am in a quad where my wife and I are dating another couple. One time my wife is talking to me and she says something like "I don't think i could share my boyfriend with another woman." (This was because someone at his work was flirting with him.) I just looked at her and asked, "Like his wife?" She got this funny look on her face because she had forgotten they shared him. She replied, "But I like her."
So your SO dating someone new is kind of scary. However, if you get to know the person, you can see them as a real person with flaws and good features. And that may make it less scary. Who knows, you may like the person he meets.
I also think it is a good idea to let him know your worries without trying to guilt him out of dating.