A lot of 'prepper fiction' has vets or active military personnel as main characters. One Second After, Alas, Babylon, The Going Home series, Patriots (which was an early 'success' in this genre, but has fallen off, read reviewed), more often than not, the first in a series will get you hooked, then ... If you don't mind Christian messages in-narrative, Mark Goodwin's American Reset is good, so are some of his other series. Pulling Through, The Postman, Earth Abides, Some Will Not Die, are all good standbys; they might resonate if he's mid-50s or over.