lunar has it. You seem to think you have the power to make or break his relationship but, even though he allegedly shagged you more than once, my money's on him being able to convince his dp that you were nothing more than an easy lay who came on strong and led him to have a drunken moment of weakness, after which you effectively blackmailed him into continuing to see you by threatening to tell her.
Imo you should tell her as she deserves to know what a scumbag he is. Of course you won't come out of it well, but maybe it will teach you not to mess with another woman's man.
As for their relationship has no solid foundation, this is pure speculation on your part of the kind which is designed to make you feel smug and superior when in fact you should be feeling nothing except abject disgust with yourself.