If my son’s wife left him 4 years ago and now he can’t get in touch with her and wants to remarry, what can he do to get a divorce and move on with his life?

In any lawsuit, and a divorce proceeding is a lawsuit, the non-filing party must be notified of the action. This is to ensure that they have a chance to be afforded the right to be heard in court.
In a divorce, the party seeking it the "Petitioner" needs to file a "summons and complaint", and have their spouse the "Respondent" personally served. When a spouse cannot be found, then that spouse can be "served" a complaint by what is known as "notice by publication".
Basically, the absent spouse is notified of the divorce by publishing notice of the proceedings. Specifically, the filing spouse will have to present proof to the court that they made a genuine effort to locate their missing spouse. They will have to convince the judge that they checked with family/friends, the DMV, voting record, etc. Basically, they must convince the court that they utilized every source that would most likely lead to discovering the missing spouse's whereabouts. Once this is done, the court will be allow the petitioner to serve the respondent by publishing notice in a newspaper as opposed to the more usual method of personal service. The judge will instruct as to which paper should be used typically one in the area of the respondent's last known location.  
Once the respondent is in "receipt" of the summons and complaint via notice in the newspaper, they have about 20 days to respond. If they fail to do so within that time, then they will be considered in "default". The petitioner may then request a "default judgment" from the court. If granted, it will done according to the stipulations listed in the summons and complaint. In other words, it will be granted on the terms that the petitioner requested.
This is just a brief overview your son really should consult directly with a divorce attorney in his area for more specific advice.