Anxious about engagement ring

so, my boyfriend told me he wants yo propose this summer, and he has two rings to choose from. one is 1/4 inch gold band that has been passed down for like 100 years from his grandma and the other is a platinum 1 carat kite cut diamond from his other grandma. he wants to somehow use both but I'm a little nervous on how that's going to work. I'm afraid that the two might not fit on my finger if he uses both for engagement and wedding band. also mixing metals scares me. and I don't want him to change the gold one because it's been the same for literally a century. So should I tell him my concerns or just let him figure it out and trust him to make the right call?