J.H.
I would simply write him and say "You know that my mother was always saying she wanted me to have her jewelry when she passed. I don't care about any of the jewelry except for her wedding ring. I would appreciate it if you could have her wedding ring ready for me to pick up on *insert date*. I will be there at *insert time*, and to pick up her wedding ring and only her wedding. Thank you!"
I think making it very clear what you want (the wedding ring only) and then setting an exact day and time is the best bet.
*hugs*