Do you love him?
Really, truly, madly, deeply?
Do you want him?
Do you want to spend the rest of your life with him?
Are you only willing to let him be with someone else because you love him so much that you consider his happiness to be more important than your own?
If so, then tell him all of that.
He needs to know.
Otherwise he may think that you do not care much whether he stays with you or goes back to his ex.
Of course, only he can make the final decision, but he needs to be in possession of all of the facts.
How would you feel, if it were the other way around, and he just said to go to another man, if that is what you wanted to do?