It doesn't appear you have a choice in the matter. It's not your airplane, and it doesn't belong to the pilot who is flying it right now, either. You don't work for that pilot, and he doesn't have the option of hiring you. You work for the company that holds the 135 certificate, ergo you do whatever they tell you to do. The pilot flying the airplane will also be employed under that certificate, and he will fly with whomever he is told to fly.
Therefore, you have no say, and neither does he. He may have some influence, but from your comments this appears to not be the case.
While it's a big juicy carrot dangling before you, remember that at this stage you're in the infancy of your career. The chances of a position coming up for you are typically very slim, and like most of us in the industry, you have a long way to go. Be content with having a job. You'll see a dozen more opportunities like this come and go, perhaps many more than that. It's just the way things go.
Therefore, you have no say, and neither does he. He may have some influence, but from your comments this appears to not be the case.
While it's a big juicy carrot dangling before you, remember that at this stage you're in the infancy of your career. The chances of a position coming up for you are typically very slim, and like most of us in the industry, you have a long way to go. Be content with having a job. You'll see a dozen more opportunities like this come and go, perhaps many more than that. It's just the way things go.