MY opinion is that is a question for a individual and he should get individual guidance. As for some factors that perhaps would come into play.
1. how long has the boy been dating(more than 3 years). the longer he has been dating the more unlikely it is for him to be willing to wait.
2. does he usually have names or does he have some difficulty in shidduchim. (yes there are some boys who have difficulty).
3. is this much younger girls so uniquely appropriate for him or he just hasn’t heard another name in the preceding two day.
and I’m sure if I thought for 5 minutes there would be other factors as well. So as I said it’s a very individualized question.