Yeah yeah yeah I get that he's a dragon and he's technically ageless or whatever. But since his first form was of a child, he acts like a child, he's seemingly going through adolescence like a normal human (or maybe faster. idk i cant tell how much time has passed) and acting like a jealous teenager. If it looks like a child and acts like a child but "hyo-un's not human so it doesn't count." It's giving the same energy as "b-but the loli is 1000 years old!!"
Ofc I don't think taehyuk's intention is to groom him, but I do find it sketchy that he's basically raising hyo-un as a guardian/parental figure and he will later develop a romantic/sexual relationship when hyo-un's takes on an adult appearance. It's like the usagi drop manga all over again.