Welcome to our forum.
I see that your head is in favor of the polyamorous ideal, however your heart is in pieces. I think it will take some time to figure out the precise nature of these insecure feelings, but one thing we should say right off is that, yes indeed, something will be lost in your relationship once your partner progresses with this new relationship. Namely, the monogamous nature of your relationship will be lost. The words don't make it sound like as much of a loss as it really is. There is a grieving process here. You will have to be patient with yourself as you "put the monogamous dream" behind you.
I think it's possible that you may be (subconsciously) afraid that O will love F more than he loves you. Something about the threesome set-up allows you to see first-hand that O loves you as much as ever. But when he's off somewhere with F, and you're at home, you're left to your imagination, and your imagination is "treating you badly." It is telling you that you don't "measure up to F," and O is going to love F more than he loves you. Somewhere in that subconscious could be the fear of abandonment: "O won't want to be with me anymore. He'll want to be with F instead." And there's always the feeling of being left out: "Here I am babysitting our son alone while O goes off and has fun with F." It's uncertain which of these hobgoblins are haunting your subconscious, but it could be all of them, and then some I haven't thought of.
It would probably help to have some deep heart-to-hearts with O about your feelings -- not blaming O, not blaming yourself either -- just letting the feelings out, talking about your wants and needs, and negotiating what can be done to meet those needs. For example, can O babysit the youngster while you go out and have some fun?
It might help to develop a relationship (platonic or romantic) of your own with F. If you feel like F is your friend, you will feel like he is on your team, and doesn't want to hurt your or diminish you in any way. You'll have to think about whether this is something you could try, and talk it over with O. Maybe O could babysit the youngster while you and F go out (for dinner, a movie, whatever you'd like to do).
These are just some foods for thought. Some of them might help you; some might not help or might just be "covering the same old ground." But hey, even if one thing helps it's worth it, and even barring that it's still a way I can let you know that I sympathize for you in your situation.
I've found in my own experience that when I've had jealous feelings, it's usually "all about me;" that is, it has to do with me feeling like my needs aren't getting met, while others' needs are getting met. If I feel like my needs are getting met, I feel much more generous and relaxed. So we need to tap into your mind to find out what needs aren't get met, or what fears are being stirred to the surface.
Hope I can help. Glad you could join us.
Love means never having to say, "Put down that meat cleaver!"