He's a 'bubblehead'?
(Navy slang for a submariner.) The submarine service is also known as the 'silent service' and for good reason: they stay down below and literally become one with the ocean. Get found, you die.
It really depends on what type of submarine he's on. If he's on an SSN (the fast attack boats) they could be gone for months at a time, on missions so highly classified that he can't (and won't) tell you anything about it.
If he's on an SSBN (the Trident ballistic missile submarines) the patrols are more regimented; they have two separate crews, called Blue and Gold; while the Blue crew is on patrol, the Gold crew is ashore training, on leave, etc. SSBN patrols ('boomers' in Navy slang) last approximately 80 days.
Become interested in the Navy. Learn to read a military person's uniform: ranks and rates, ribbons, medals and decorations, and other insignia. The military uniform, to the initiate, will tell you the rank, unit (sometimes), awards and decorations he or she earned, as well as qualifications.
Take notice of his 'dolphins' (the submarine qualified insignia); a LOT of hard work went into making him qualified to wear them; while he's learning to become qualified, he gets mercilessly razzed by the other already-qualified dolphin wearers.
Be patient; if he's on patrol, he may not be able to answer e-mails. I don't know if they still send 'familygrams', (brief messages sent to the submarine by family and friends) but subs nowadays can pick up and send e-mail when operational requirements dictate.
Hope this helps.