What Ring size would an 48 year old wear?

I'm trying to find a gift for my boyfriend . he is 48, but I don't know what is his size . I don't have time to get his finger measured by a pro but maybe I can? Please don't Respond with just go to a jeweler. Also do you think she would like an infinity ring?

P.S. he is height is 5ft 7"
He weight 147lbs
He shoes size is 10

There is no way we can tell his ring size.

Who is she? How would we know if she would like an infinity ring?