It sounds to me like he may want to play as the USN. In that case, Silent Hunter 4 is the way to go. Fully patched and with mods like Trigger Maru, it is the premier Pacific submarine simulation. You can read more about it on it's forum here:
http://www.subsim.com/radioroom/forumdisplay.php?f=202
If he wants to play as the Kriegsmarine in the Atlantic, I would recommend Silent Hunter III. It is a bit dated in the graphics department, but with mods like The Grey Wolves, it is as good as you'll currently get in the Atlantic. It's forum is here:
http://www.subsim.com/radioroom/forumdisplay.php?f=182
Silent Hunter 5 is currently more of a work-in-progress. The modders will make the game great, but we're not even a year after release. New mods will come out that will replace old mods, and swapping, checking and updating will be required. If you don't think that your stepfather would like to do that, then consider Silent Hunter 3 or 4, as they are generally fully developed.
Also, 5 comes with a DRM requirement that you must be online at all times to play. While not particularly problematic, it may prove a nuisance for someone of your stepfather's age. Silent Hunter 3 and 4 do not have this type of protection, and one may simply click and play regardless of internet connection. The system requirements for Silent Hunter 5 are also very steep, and the game may not play well on a laptop. 3 and 4, however will be well within the requirements of the system that you described.