I believe he can gain an exemption if he repeats Korea's gold medal exploits at the Asian Games in 2018. That's probably his last chance, tbh - I imagine he'd be straight off to do his service if they failed to win in that competition. It leaves us with a bit of a conundrum next summer - do we keep him on, and stake everything on him winning the Asian Games in August 2018? If so, we'll also have to prepare for the likelihood of South Korea not winning, in which case we'd be paying him for the two years he'd then be absent for, with his contract expiring in 2020 at the conclusion of his service. He'd be back by the time he's 28 or so if everything goes well and nothing untoward happens, which should be about when players hit their stride anyway - so if we give him a new contract when he leaves that extends his commitment to us past 2020 (factoring in his service period, I'd imagine he wouldn't ask for wages as high as he would if that wasn't necessary), we could in theory get him back by 2020.
Or, we could decide that it isn't worth the risk and sell him next summer - trouble is, firstly, I imagine teams that want to take him will be faced with the same conundrum, so we wouldn't get as high a price as we would given that service period, and secondly, that's a pretty bad way to treat a player, binning him just as he's faced with the dark prospect of missing out on two productive years due to his national service commitments. Same goes with releasing him or something in 2018, if it comes to that.
I'd like to think that, subject to his form staying the same into 2018, and in the eventuality that the South Koreans fail to win the Asian Games, we'd arrive at some sort of gentleman's agreement with the lad where we pay him reduced wages during his term of military service and then pick him up again after it. But, those are the sorts of things Levy handles with considerable aplomb, so hey, not my place to judge.