He has a pretty short attention span so I want to show him a drama that will get him hooked immediately and want to sit through 16-20 hours watching it. He's watched Korean movies before like Train to Busan and he's open to all genres.

I'm thinking of showing him either DOTS, Goblin, W, or Healer. What do you guys think?

i think w and healer are both really good choices bc they really keep ur attention. also maybe a thriller type drama like signal or bad guys or liar game or gods gift. . i've shown those to other people who aren't kdrama fans but they really liked them. like my mom really liked signal and gods gift. my sister liked healer and age of youth and it's okay that's love and masters sun. i think goblin could be a good choice since her already saw gong yoo on train o busan, but imo the show kind of was slow a bit so it may not be good for someone with a low attention span. i hope u find something he would like????!! good luck!
I really liked Pinocchio. It's a decent romance that shows the dark side of news journalism which was pretty great.
Circle, Signal,Voice and W ^^
Heartless city, Bad guys and Signal 
Healer is a good choice. Same with those which are more of the thriller/action genre, such as Iris, Signal, Tunnel or Voice.
Boyfriend? Say no more!

Healer
City Hunter
K2
Signal
D-Day
A Beautiful Mind
Good Doctor
Voice
Tunnel


Optional if you later want to test the waters:
The Girl Who Sees Scents
Kill Me, Heal Me
Pinocchio 
Master's Sun
Bring It On Ghost
Descendants of the Sun


signal (it got me here )
I think Signal would be a good start for a guy, but I hope it doesn't set his standard too high from the start XD
My fiance loves dramas beause #Goblin and he loves Circle too!!
Hello Monster/I remember you