You're going to put your demand request in writing 
Big mistake, honey. You'll be giving the fucker written notice that he has yet more time to come up with an edited, sanitised, version of his insalubrious activities and he'll have more opportunity to rehearse and practise the minimalist confession that he'll eventually make.
If you want the get the truth out of a devious, lying, cheating twunt always take them unawares and watch their body language.
As fiventhree has said, doing the opposite to what he's expecting may elicit a breakthrough. If your conversations on this vexed subject have been heated less than civilised, back off and adopt the resigned and regretful demeanour of a martyr.
Quietly tell him you're resigned to the fact that he will never be honest with you and that, as you now need time to consider whether you want to stay married to him, the subject of his duplicity is out of bounds.
And then, equally quietly, tell him that regretfully sex between the two of you is off the menu until such time as you've been tested for sexually transmitted diseases.
Keeping a straight face add that you haven't felt entirely right 'down there' for a few months and, having googled the subject, you're aware that some stds are symptomless for males and it's entirely possible that he's transmitted chlamydia or another disease to you that he's unaware of.
Follow this with the silent treatment over the weekend - briefly engage in any conversation he initiates on other subjects interspersed with pensive looks, gazing into the distance, the occasional deep sigh, and generally emanating a preoccupied air as if you're wrestling with a 'big decision'.
If you're engaging with other adults in his presence be especially animated and act as if everything in your joint garden is lovely, reverting to martyr when you're alone with him.
If he runs true to form, he'll crack by Sunday evening and his first hesitant genuine admissions will start to push through the dark soil of his deceit like spring bulbs but, unfortunately, they won't be anywhere near as pretty.