I'm a girl who mainly watches romance, but here are some that he might like:
- City Hunter (i actually got my dad to watch this with me and he liked it)
- Bad Guys (a great action/crime/thriller that's short - only 11 episodes)
- Signal (it's similar to the american movie Frequency, good for beginners)
- Tunnel (this is actually one of my favorites)
- Stove League (you don't need to like sports to like this drama, but bonus if he's a fan of baseball. my favorite drama of 2020 so far)
- Hello Monster (another detective/serial killer drama since you said he showed more interest in tell me what you saw)
- Circle (if he likes sci-fi, this was a good 12 episode one that i remember liking)
- Memories of the Alhambra might be good to show him early (imo the ending sucked, but that might not be a problem for someone watching for the first time since he doesn't have anything else to compare it to)
- If you mainly watch romance and want to pull him in that direction, I would recommend maybe Pinocchio, The Master's Sun, Healer, or You're All Surrounded
others:
- Rugal (currently airing now but it's pretty good so far, on Netflix)
- Memorist (also airing right now, but i'm liking it)
- Nobody Knows (airing right now, i haven't watched yet but heard great things)
- Eulachacha Waikiki (i haven't watched it yet, i don't watch much comedy but this is on my list)
- My Country (if he was interested in the historical aspect of scarlet heart, i thought the first two episodes of my country were stellar, i just was in the middle of watching six dramas so i had to put it down and i have no idea if the ending is good. but you could give it a try!)
- Voice (another jang hyuk thriller)
i would not recommend showing him dramas that are based on american shows like criminal minds or suits if he has seen the american version because what happened with me is i kept comparing them to the originals and they were never as good so it killed my enjoyment
movies might also be a good way to introduce him:
- Parasite (obv)
- Train to Busan (obv)
- The Pirates (one of the first kmovies i watched, i thought it was hilarious)
- Rurouni Kenshin (based on the anime/manga of the same name, the whole series is great)
- Kingdom (another japanese historical based on a manga, it was a fun/easy watch)
- Luck-Key (comedy, entertaining)
- Twenty (comedy, it's been on my to-watch list for ever but the cast is good)
hope this helps!!