Hi Banb! Sorry you're in this sitch, but there are many people who can help you here.
If the inviting your H to go out to lunch, or pick things up, etc. isn't working, then I would change course. Look, he's ensconced in an apartment, with a OW in her 20's, who hasn't had much life experience, still innocent to the ways of married life, with children, jobs, and the whole pile that goes with it. I am assuming that to him, this is fresh and different, and very appealing. And she probably thinks she's hit the jackpot with an older guy, who has money to spend ... on her. I am making a lot of assumptions, but since he's not communicating with you in any real fashion, I would say that he is infatuated with her at this point.
So, what to do? Not much you can do, except go dark, do not contact him unless it's to do with finances, business stuff or the children. Protect yourself re finances ... make sure he has no access to large amounts of money that he can spend on his OW. Give him space and time to make a fool of himself (I mean, 54 vs 21 ... she will have far more energy than him, and if she can cheat with him, a married man, I am sure she will have no problem cheating on him). Don't answer the phone if it's him ... you can text him later to find out if it's urgent. In the meantime, take care of you ... GAL, have fun, do not let this sitch get the better of you.
Whether this is MLC, only time will tell. Having an A with a young person could be one of them. Has he changed his clothing, work out more, hairstyle change, to fit in with her and a younger him. Was there a negative event recently? A death in the family, perhaps, or a change in career path? It takes time to get any clarity on that.
Take care, and good luck.
Me:57 H:52 M:28 Got another lawyer last year and filed. D35,S/D twins28,D22 EA4/04 End? Who knows? "Life is like a mirror. Smile at it and it smiles back at you." — Peace Pilgrim