How to do it?
act cool. No you aren't feeling cool. Anything but, this hurts like you are called towards the sun and as if he can turn your life into glorious vibrancy. But you know better at the head level. It's an illusion.
so ... maybe think about it and plan how to deal with it and your own need for him. (btw, he's playing on that need).
When you speak to him, keep the language moderate. Job focussed. When he texts you, leave time before replying. At first it might only be half an hour when you could have replied earlier. As the days progress, move that to 3 hours; a day ; 3 days.
If he questions why you aren't immediately available emotionally, then say that you have other thigns going on. If he doesn't ask what; good. If he does ask what, say "tell you later".
If he says "can't wait to see you" say "me too, when it's possible". The thing about saying 'when it's possible' is that you are showing that it's not possible ... and you're not immediately jumping to him. Move to "I'd like that too". Not "i'd love to"... I'd like to.
Setting up expectations of time is very strong. Offering you a time to meet sets up an expectation; be aware of what's going on. Control your response to that. if you are simply not able to say No, then try to change the time of the meeting. That's exerting tiny bits of control that you can trust yourself in.
Don't arrange business meetings one-to-one. Don't text him suggesting a meeting.
This is all about managing your heart. You won't get it right at first, you'll fall on yoru face. But you can practise. This man does not have your best interests at heart, or he would not risk your husband's loss of regard for you. If he was truly in love with you and decent with it (it does happen), he'd go about it in a very different way.