My dad was not my biological dad, but I do consider him my father.
The only thing that puzzles me about this story is the sequence of events.
1. Couple divorces, he agrees to pay child support.
2. Ex-wife sends a lawyer's letter five years later, asking for more money and a reduction of Dad's custody time.
3. Dad then decides to ask for a DNA test.
His only beef should be with his ex, not with his children.
Whether or not he's the bio dad, the kids should still be his primary concern.