I suggest having a read of some of the literature available about ethical non-monogamy and looking at some of the other places online that are focused on discussions about it for a broader range of input than you'll get on Mumsnet.
You are not in an ideal position to turn your existing monogamous relationship into one that is ethically non-monogamous in some way, because of where you've got to - it doesn't seem to be a free choice, but rather one you feel forced into. It may be you try it out and end up separating anyway.
Would you rather formally separate and work on establishing a friendship/co-parenting relationship that works for you both, rather than try out ethical non-monogamy?
It's often hard to tell how things will feel before they happen - I quite often find the things I feel worried about in advance are OK when they occur, but I trip up on things I hadn't expected to be a problem. How do you think it would feel for him to have another partner? It's worth writing this down, thinking about both broad feelings and specifics. I do polyamory and most of the time when I think of the person I'm involved with being with somebody else, I feel happy he's getting to be with someone he likes, doing stuff they like doing. This concept is "compersion" - and I often think of it as "vicarious happiness" - being happy because someone I care about it happy. The times when I feel less like this are when I've not seen him for ages and I want to and it's not practicable for us to do that any time soon [our relationship has never been one where we live together and when we got involved with each other he was already living with a partner and seeing other people, so a lot of the context for you will be different] It's important to consider your living space and boundaries for you both around how other people use your home. It's reasonably usual within ethically non-monogamous relationships to decide it's not OK for another partner to share the bed you use together, if there is one [and from the perspective of a hypothetical partner for him, it's unlikely she'd want to do that, either] It's also not unusual to decide you don't want to overhear your partner doing sexual things with another partner. Particularly as you have adolescent children, it wouldn't be at all unusual to decide that you need to move slowly with any new partners - introducing them at an appropriate time etc.
Think about boundaries - where you end and where he begins. You can't stop him having another partner, but you could divorce him if he did. You can't make him stay with you but equally, he can choose to stay with you if that's what he wants to do. This is important when you think about rules/guidelines etc for how you do things; you can't control somebody else's behaviour, but you can control your responses to it.
I think the TLDR version of this is: ethical non-monogamy is possible, but it will be really hard work. Is it something you want to try, or do you want to separate or are you both happy remaining married as you are?
Couples counselling from somebody with experience with ethically non-monogamous/polyamorous relationships might not be a bad idea for you to explore all of this further. Pink Therapy www.pinktherapy.com/ is a good place to go looking for somebody suitable.