ArmyVet your son should certainly pursue all possible paths to a nomination. I believe the above posts, identify all the sources. MOC and Senator policies do vary and yes many only will allow one nomination. Additionally, some MOC and Senators coordinate so that you are effectively limited to one nomination from your state.
As ca2midwestmom has suggested your son should reach out to whomever coordinates the nomination processes for your Senators and Representative. Hopefully, they would share with him the details as to their current nominating practices. That is both any limitations and coordination which goes on as well as their typical characteristics of the slate they pass to the SA. That is: principal nomination, ranked slate, or unranked slate. Although, as mfrey points out you may have to wait until after an election for the relevant representative.
In addition to focusing on the sources of nominations your son should certainly be working on whatever he can do to increase his chances of receiving a nomination. I may be going a bit off topic with the following. That is investigating those things which can increase ones chance of receiving a nomination. For example, in some states Boys/Girls State may help significantly and could be an effort he would need to make during Junior year. You might want to investigate that now for your state to plan for the coming year and his seeking in state nominations. Here's a thread relating to that subject:
http://www.serviceacademyforums.com/index.php?threads/boys-state.47891/#post-474964
That post included a link to a RAND white paper/study.
http://www.rand.org/content/dam/rand/pubs/research_reports/RR700/RR723/RAND_RR723.pdf
The Rand report, particularly pages 8 through 10 provide insights, at least at one point in time and for a specific SA as to how they factor different aspects of a candidate's profile into a WCS. However, I suspect the SAs are more likely to be similar than dissimilar in their practices. So this probably provides insights which are relevant to all SA.
In the case of my own DS, as a sophomore he was initially concerned when he viewed SA incoming class statistics and feared that one had to have many more sports and extracurriculars than those in which he was participating. However, if you look at the pie chart on page 9 of the RAND report, you see that at least with West Point at that point in time athletics did not supersede extracurricular activities, faculty appraisals, or the CFA in importance. Additionally, the weights of different activities may provide some insights as to how to prioritize potential choices.
Lastly, while not typically part of the nomination process you should encourage him to train for the CFA early, Junior year, and run the whole sequence periodically. It is the one part of the WCS which is almost completely under the control of the candidate and pretty straight forward given appropriate training time for a candidate to do well on.
Good luck to your son.