Is Jason Holliday, protaganist (anti-hero) of the Shirley Clarke
film, Portrait Of Jason, still alive?  If so, how does one go about
reaching him?

I can't give you a definitive answer, but considering that Jason (born
Aaron Paine) was a 33-year-old gay street hustler, former mental
patient and heroin addict in 1967 when the film was made, the odds of
him having survived to the age of 70+ are pretty slim. There is no
recent record of him anywhere under either name.