Anyone rushing a partner to commit to a timeline they're not comfortable with isn't the right person for them.
Three years might seem a long time for some people, but if it was your son's first long-term relationship, it probably felt nowhere near as long. In contrast to most, I can totally empathise with his position. People reach different life stages at different times, and quite clearly the thought of marriage at this point of his life was something daunting that he just wasn't ready for.
And that is completely fine. We live in a society where most people always seem in such a rush to do everything, have kids, get married, whatever, and then only find out afterwards that they're not really compatible. Rather than just take the time to enjoy each other's company and life naturally, rather than always be wondering what's next.
There's probably more to the story, though. For her to act so cold and indifferent to someone only a few months ago she was putting pressure on to propose to her, there's surely a good chance that her head has been turned.
Ultimately it's the best outcome for both of them, even if you son struggles to see it now.