Similar position like you but in reverse kinda idk. Initially my gf didn't know at all about my stutter. On our first calls she would talk majority of the time and one day when I told her. She didn't mind, she expected I was shy and knew something was off from the start. Together for 3 years now and she taught me how to deal with others who don't know about my stutter. Generally people don't care about your stutter but tend to like you more for your honesty. So many people really didn't mind and actually took a liking to me more. So let him know as soon as you can. Keeping it hidden won't do you any good and he'll question why you kept it from him. He'd appreciate that you put your trust in him in that regard. best wishes you got this