If you decide to take up this job check very carefully that you are covered by the company's employees liability insurance. You don't want to be doing something for nothing only to get injured and end up financially stuffed. It might be worth getting abit of advice from BALPA or IPA which ever one you are a member of (you are a member of one or the other I hope).
This is really tough: I would ask that there is some quid pro quo. If the boss man isn't going to pay you I would try to encourage him to offer something else in lieu such as expenses or a promise of a minimum amount of flying, or a review of the salary status after a defined number of weeks. If he's not willing to match your commitment I would shop him for breaking minimum wage, and walk away.