I am currently wearing my late Mother's engagement ring, which I love, but it is also hugely valuable, a good five figures, and I wouldn't part with it, although would leave it in my will.
I do have another ring, left to me by an aunt, which is very pretty and valued at around £2,000 and I did offer it to my son when he was with a previous girlfriend, if he wanted to give it to her. (they broke up before he did so)
It seems your son and his GF have not been together very long , eight months is no time really. and yes, 25 is fairly young to get engaged these days
How about saying that you'll give it to him after a certain period of time --18 months or so seems more sensible to me. I don't think there's anything wrong with being uncertain about his GF, or telling him that you'd like to get to know her better first.