He already has a feeling something is going on - otherwise he wouldn't have asked in the first place. I'd just let him know that another company MIGHT be calling you soon and it may be short notice. Tell him you'll do everything in your power not to leave him high and dry and that you will help him to cover any uncovered flying. Whether that's helping him find another pilot or if you can cover the shift.... If he sees you bending over backwards to try and help him he will respond in kind. That doesn't mean he'll take you up on it.