The two communities that would be most closely associated with computers are the Information Professional (IP) and the Information Warfare communities (IW) (as I kind of hinted above).
The IP community deals more with the communication infrastructure...everything in the RF spectrum to the maintenance and security of networks.
The IW community is more vast...they cover Signals Intelligence (SIGINT), military information support operations (MISO), operations security (OPSEC), computer network exploitation/attack/defense, and military deception and then some.
The IP and IW overlap (somewhat) when it comes to computer network defense. Both communities are involved in newly created cyber billets for USCYBERCOM and FLTCYBERCOM.
Computer science and cyber security majors could find themselves in either community -- it really comes down to an individual preference, something that your son would have plenty of time to explore and talk to officers within those communities.
As far as Navy billets....
(1) While a small number right now, there is an opportunity to go directly into the IP/IW community right after graduation.
(2) SWO-IP/IW option program, where officers do 1-2 SWO tours, then automatically are re-designated to the IP/IW community. This isn't such a bad thing, as you get an understanding of what the Navy is really about.
(3) Lateral transfer, applying to be re-designated to IP/IW community AFTER serving in another community. A board convenes to determine approval/disapproval (no guarantees on this one). A key discriminator (among others) in this board is having a relevant technical degree (i.e. computer science/IT/EE/etc) or being in a comparable job (i.e. ship's communication officer).
If your son is really keen on majoring in computer science, IT, or cyber...and he puts the time, effort, goes above and beyond, the odds of receiving one of these "few" billets greatly increase. FWIW, as you become more senior, you become less technical/hands-on...so the most technical your son will ever be is probably at USNA or within their first 1-2 jobs. HOWEVER, having the knowledge is something that stays (and needs to stay) throughout the career. Understanding what an IT or CTI/R/N/M is telling you in technical jargon is a lot easier when you have the background and comprehension.
Additionally, there will be opportunities to attend postgraduate education. Furthermore, the IP/IW communities are part of the Information Dominance Corps, so your son will have to be a knowledgeable expert in what all of the IDC communities (IP, IW, Oceanography/Meteorology, Intelligence, and Space) bring to the table (initial step is earning the Information Dominance Warfare Officer pin and community basic qualification).
Two good links:
(a)
http://www.usna.edu/Cyber/
(b)
http://www.usna.edu/Cyber/idc.php