NOT telling him upfront is a surefire way to make sur eyou NEVER EVER work for that particular contractor and pretty much anybody he talks to EVER again. If it were me, I'd probably go ahead and just recommend somebody to do the gig and be a Big Star in two cats books (the contractor and the sub) because for a two week run subbing out a couple of nights is a lot of trouble (for the sub AND the MD). On Broadway (and Off) there are standards/rules - you can't really start subbing until you have been ON the job for a specific amount of time. Generally the guys you are subbing the job out to have already been working on the book, they've already attended rehearsals and performances of the show (in the pit). A buddy of mine subs Broadway a lot and apparently a few conductor/MDs have DVDs of them conducting the show so that potential subs working on the book with a soundtrack can actually follow all of the cues, countoffs, cuts etc. before they even get to the pit.
At any rate, a joke I heard recently-
National ballet troupe is touring the US with their orchestra and in one small town their percussionist is taken ill. They make a frantic call to the AFof M local and get a recommendation for a jazz drummer who has done a lot of vaudeville and variety shows, can read fly ****, the whole deal. They say great, downbeat's at 7. After the show, the MD calls the drummer backstage and fires him, saying that this was the worst performance he's ever heard form a professional musician. The drummer says "What's the big deal, I caught everything!?!"