Should I let him have the ring?

My son wants to pop the question to his girlfriend of 8 months. I think he is a bit young - 25 - but he's in love and we've all been there. I've only met her three times but I found her very...aloof. She didn't say anything I can put my finger on but I got the distinct impression she thought she was 'above' us. I can only hope they need a long engagement so that everyone can get to know each other better (or dare I say it hope that my son might change his mind - said it now!)
Anyway, my beloved mum left me her ring when she passed away years ago and it was always a family joke that one day my son might like to use it as an engagement ring. Of course at the time he snorted and said he'd never get married but now he's come asking for the ring and I don't know what to say? I can't bear the thought of this young woman wearing it. I don't think my mum would have wanted her to... What do I say? For now, I've said I have it stored away for safekeeping but will try look for it. Of course I know exactly where it is.