Henry Hopper -- Dennis Hopper's Son Accused of Raping 15-Year-Old Girl

Dennis Hopper's 21-year-old son has been accused of raping a 15-year-old girl ... after plying her with drugs and alcohol ... this according to a lawsuit obtained by TMZ. Henry Hopper -- who starred in Gus Van Zant's "Restless" in 2011 -- is being…