Welcome to FT!

As a general rule, last-minute tickets are expensive. The least expensive tickets have advance purchase requirements. They usually go away one or two weeks before the flight, even if they haven't all been snapped up by then.

The airlines reason that people who show up at the last minute usually have to fly, no matter what, and will pay whatever they must if they have no choice.

Your son's willingness to stand by for a flight with an empty seat doesn't change this. Most last-minute travelers will if necessary. They still pay top dollar.

My suggestion is to buy a ticket now. Winter travel to Hawai'i is always in demand. Prices are unlikely to get any lower.

BTW, you mentioned his going to Hawai'i but not his coming back. The airlines also treat people who buy one-way tickets as "cash cows." It may be less expensive to buy a round trip, "returning" midweek a few weeks later (price a few different days), and not use the return. This is theoretically against the rules, but AFAIK airlines have never come after an individual who doesn't make a habit of doing it.