Building an addition up or out is far more cost effective than digging an existing crawl space. The crawl space must be manually dug and must avoid undermining the footers of the house foundation. On top of that, you have to move all the spoils to the outside and transport it to a disposal location. This is an unbelieveable amount of labor. Not only that, but when you are done you will have added almost no resale value to the house. In most cases, basements are not counted as living space square footage in value appraisals.
The labor costs for such a project would be tremendous, and is not worthwhile for most individuals that are gainfully employed doing things besides digging dirt. If your son has about 5-years on his hands while he becomes a miner and digs out a crawl space, you should probably encourage him.
The advise is, if you must have a basement, buy a different house. If you are looking for more space, build an addition.