Billy Bob's Son Alleged Sex Crime Victim

Law enforcement sources tell TMZ Billy Bob Thornton's son is at the center of a criminal investigation -- the alleged victim of "unlawful sex."

Here's what we know. Willie Thornton, age 14, was dating a 22-year-old woman. Her ex-boyfriend apparently became jealous. Sources say the ex called the LAPD, informing them his former GF was having sex with a minor.

Law enforcement sources tell us there is an "active investigation" into the crime of "unlawful sex" -- translated, an adult having sex with someone under 18. We're told Willie is cooperating with the sex crimes unit. And, we're told, cops have also interviewed Billy Bob.