Engagement Ring dilemma

So my grandma just gave me a family heirloom. She gave me her mother's engagement ring and wedding ring to wear when I get engaged/married. My boyfriend and I have been dating for 2 years and we've definitely talked about getting married but don't know when we plan to do it, so how would the ring situation work since I have the engagement ring? I don't want to give it  to him until we know for sure we want to get married, but if he were to ask for the ring when he plans to propose then obviously I'd know he's going to propose to me but I want it to be a surprise. Should I have my parents hold on to the rings so that he can ask for it without my knowledge when he's ready to propose? Or when he's ready should he just ask me without the ring and I put it on later? What are some ideas for this?