I think is his son. That was explained in episode 19
The uncle is lying the dna test burned was with sangjun and the child and the result was not father. He was holding his laugh in the scene sangjun mother talks about it in the flashback, he is using her naivetee to make money with the fake child
has a son, and is from the mother, so the new theory is that she is a delusional fan xD