If you are firm in your decision, how long do you think he will continue to hassle you? Perhaps taking the approach of 'this is something I really want to do, and since I'm the one who'll have to make the change and deal with the fallout, don't you think it'd be better if this was my decision? And I'd really like your support in this. Thank you' might work?

I can see how you really need his support to continue.