I was there about six months, mostly in Seoul, a bit to the northeast in Gangwon-do. I never once felt in danger or saw any crimes..by natives anyway. That's not to say everyone's a saint, but he's more likely to find trouble in a bar on base than outside the gates. If he was female I'd say watch out for skeevy dudes (women's rights are a big issue, and there are numerous famous cases of like hidden cameras in bathrooms, creepy dudes attacking or even killing women for ignoring them/not dating them/whatever), but as a guy himself, as long as he doesn't make trouble or hang out with someone who does, he'll be fine. It is a country known for protests, but he'll be told to stay away from them and the areas where they'll happen. Historically there have been a few violent ones, but it's been a few decades (maybe the last was the Gwangju uprising in 1980?). I was there for two, a larger one that occurred after a ship carrying school children sunk and killed them all, and corruption in the government was part to blame, and then I was there later when taxi drivers went on strike for more pay. Both were peaceful. On another note, the commissary on the largest base near me had a better variety of food than any I've been to here (that may depend on where he's stationed).
I personally loved South Korea. I've gone back on my own time a number of times. Seoul itself is a very cool city. The cost of living is similar to here overall, but the food and hotel prices are better, so as a visitor it's very nice. Korean food is certainly not western, but westerners all seem to enjoy it. Even if he doesn't, did you know there are more Starbucks in Korea than here? There's plenty of burger joints and all that as well. The public transportation also is amazing. I'm talking like a bus foe each route comes every 5-10 minutes, super cheap taxis (like <$10 anywhere within reason), and a great subway system. The people I know in the city with a car have it because they drive to visit family in the country, not because it's more efficient or nicer than taking public transportation. As they rebuilt Seoul and the surrounding suburbs, they city-planned very well; there's walking paths, hikes up mountains, lots of parks, etc., everywhere, so as someone active, there is plenty of new things to try. Being stationed there will be a treat (assuming his command/det is fine). The countryside is very beautiful, and it's worth getting out to visit temples.
On knives, I suggested what I carried. My Dragonfly Salt was always on me since I could carry it on and off base without breaking any laws. The humidity is killer in the summer, so it's hard on even stainless steels. There's also lots of snow in winter (fun fact, the Korean War is when insulated boots were invented, or so they told us at boot camp anyway).
Edit: And one more thing, we make a bigger deal about North Korea than they do. War/North Korea is not something that people care about or talk about regularly. It'll be in the news if there are talks maybe, but it's not something the average Korean thinks about at all. It's definitely a bigger news item here than there (and there it's more about the desire to some day once again reunite the peninsula).
Edit2: One more edit. Korea is a very monoracial country. I'm a white dude and never had trouble with anyone. The only times were when little kids would look and stare and say "Hello" or "Hi" to try out their English...no big deal. No adults ever treated me any different. I have a friend who was in the Navy stationed in the South and is half black (looks black) and said he felt he was always singled out. I would guess racism happens more in the country than Seoul, which is very modern and quite westernized. Where it happens, it seems to be aimed at black skin, southeast Asians, and Chinese. Whenever there is a disease breakout that could come in the country, it seems like flights from Africa get shut off. That said, there are also famous black folks in Korea. Southeast Asians face some racism because farmers basically/practically import brides from there because no one, especially women, want to stay in the country and farm when they can get to Seoul and make a good salary. Chinese face some racism because they are known as going there and buying up land/property.