In this situation, I would want to mention that poly might be important in the future. The earlier you mention it, the easier.
And if he puts an ultimatum to you: mono or nothing, the sooner you face that ultimatum the better for the future.
And if he leaves you totally, feeling you are dodgy for ever even thinking of such a thing, then that is part of who you are, and the sooner you know the better.
But none of that is why I would tell (even though all of that is true).
For me I would say so as soon as possible so that I do not have the feeling that I am keeping something back. I do not want a relationship where I have a secret, as secrets kept over time tend to feel like guilty secrets. In short, I would tell her (it would be a lady caller for me) I would tell her as soon as possible, for my benefit, not mainly for hers.
Just my take on it.