I don't expect to get paid by my students but I do find myself regularly needing to give them rubber bands, fuel, props, clevis's or the like. It would be nice of you to make sure your lessons are at least NOT costing him anything. I'm not saying not to borrow but do replace what you borrow. Sooner or later you will be labled a mooch by others if it goes on too long. You can also be the one to bring coffee to the field for him in the morning, don't forget the cream.