Kivyx

There is a scene where he is coughing up blood in the bathroom, tends to be the go to things to show illness.

With what was shown in the series (not seen movie) this is what we can surmise. Ki Jin was quite seriously ill. Most likely he gave up the zebra offer and "dissappeared" for treatment and when he finally was well enough he returned and made his own way in the fashion industry without selling out. And that's when he contacted Yeong Woo again to "reignite" their relationship.