If he is for real you can tell him the truth. I am not going to sugarcoat anything. Some will be scared off by this. Those are the ones you are better off without anyway. The ones that are truly worth it will stay and will not be deterred.
Long before we had actually even met in real life, at a very early stage in our relationship my gf confided her own SI to me. I know she is still doing it and not likely to stop any time soon. But it was never even the tiniest bit of an issue between us.