But the question was about biological sons.
Pre flashpoint Bruce is more than 20 years older than Jason, which makes it possible that he is his (or Tims) father, but only 15 years older than Dick which makes it unlikely that he is his father.
in the New 52, Bruce is roughly 10-12 years older than Dick, which makes it next to impossible that he is his father, and maybe 16 years older than Tim, which makes it still very unlikely.