Is he a gym blogger or something? Or is it career related? Why would he not treadmill in the gym?
I'd sit him down (or even, write him a letter) and say:
- I need some time off too. You take 20 hours a week at the gym and I barely get time to have a coffee. We are going to need to adjust that.
- It feels like you've checked out from this family. I need you to step up more both in the home, with the kids, and with us.
- When I raised a valid concern with you before you told me I was being stupid rather than looking at things from my perspective and working with me to fix things in a way that I feel reassured and that we are both happy with. That is not ok. And I will never tolerate that nonsense again.
- Let me be very clear, you need to step up and get with the program. Because I don't want to hear 'the end of the relationship came from nowhere' if you don't.
- I'm asking you now to quit the gym (for now) and be home after work in future, no...excursions until our relationship is back in track. This is a non negotiable. It's separating you from your family. You can always exercise on the treadmill here. It's us, or the gym.
Absolutely give him the ultimatum.
The only way a man is choosing not to quit the gym for a few months for his family, is if it's not about the gym but about the women he meets when he says he's just at the gym. It's a fair ask op, don't think it isn't.
If you can't give him the ultimatum, then you need to start taking time for you just like he does. And all the other stuff about him being present for his family needs to be addressed.
He'll accuse you of thinking he is cheating. In which case all you need to say is 'well your actions have gone nothing to reassure me to the contrary. But if you are or, you're not - why would spending more time with your family for a few months be a problem?'.
'Let me make this clear, you've failed me and you've failed the kids. I'm giving you a chance to fix it because I love you. But if you don't want to, then you can live at the gym if you like, because we are done'.
And before someone says if the roles were reversed it would be controlling to give an ultimatum, no, controlling would be if they'd given you no reason to mistrust them or, did everything the could to be transparent and you still weren't happy and thought you could tell them what to do. This person has taken the piss and trampled your boundaries. No one is owed 20 hours at the gym whilst their partner is shafted with no free time. And certainly not if they appear to be acting nefariously and then, calling you stupid for noticing. You have every right to give them an ultimatum because it's about the treatment you will accept. Especially because they are also fucking up their relationship with their kids too. They can take this last chance you are giving, or, leave.
Get your ducks in a row.
Make sure all your money is in an account with only your name on it before confronting him. If you have joint accounts, take your share out so he can't clear it.