Unless he goes into battlefield chasing tanks and dodging grenades. The Red Sub would be fine. There are ample supply of replacement parts and watchmaker around to servicee the watch to keep it ticking for a long time. However do check the reference of the watch. As with vintage rolex there are a lot of ingenuine example laying around. Check the serials on the watch, the caseback, and the papers. Make sure everything matches.
You won't regret it I'm sure
unless you getting into more debt that you can handle. Otherwise I would recommend the 1680.
As for retain value ...
Well that's like asking will General Electric shares hold its value?
Simple answer: Yes but life is not that simple so the answer you need to hear is after 15 years that's strong chance that your Red Sub will sell more it original price but shorten the time to say 3 to 7 years I really don't know? Maybe it won't.
From my personal experience of buying watch with the thought of retaining value or investment yadi yadi yada ... Its just that we don't want to feel like a sucker after putting a significant amount of money into the watch. Now that's a mental trap we often fall into.
If you husband and you are going to enjoy this watch and have alot of wonderful memories with it. What's the price on that? Better yet what is its value drop to zero? Did you lose out? No the memories are priceless.
Here is a link you can get yourself more familiar with the red sub
rolex.watchprosite.com
Best
Tyler