The Imposter Father

Situation (Starting Episode)

One peaceful morning, a truck was slowly gaining speed on a quiet highway. But in that instant, the truck lost control and crashed violently into the guardrail, causing a major accident. The driver sustained severe facial injuries and was immediately taken to the hospital. The medical staff had to prepare for major surgery requiring general anesthesia to save his life. The hospital urgently contacted the patient's son, who was listed as his only family member. "Your father has been in a serious accident. He needs surgery. We need you to deposit 30 million won to proceed with the treatment." However, the son who received the call showed an unexpected reaction. "That's not my father!!!" He vehemently denied it. The hospital conducted a DNA test to confirm any possible errors. And the test results were astonishing. 99.9% DNA match. Medically speaking, this person and the son were indeed related. So why was the son so sure when he said, "That's not my father"? (They live in the same house and don't have any particular grudges against each other)