First of all, storywise we dont need a new character who will be a burden to Shioon. There are enough people around Shioon to cause him enough disruptions. In the end, it can only serve to drag the story.
Second of all, why did Shioon agree to teach him, feed him valuable drugs etc without even knowing the kid's character? The kid just wants to get stronger. The solution Kang Sera offered is way more plausible than introducing a hapless kid into murim and potentially ruin his life.
Third of all, first two seasons supposedly made Shioon steel his heart and turned him into an authentic Murim person after all the hardships and losses. That naive Shioon is supposed to be no more. It's time to become the ultimate martial artist to go against Kaiser. But guess what, Shioon wants to play teacher before he heals himself to go against GMR?
Last of all, if you want to make the little guy the focus of season 3 at least make him work for it. Don't make him taller and buffed in less than a month. If you want to do this do it right.