Here is the twink - Nicholas Hoult - funnily enough another English actor born not many miles from CF.
(the best pic available as the YouTube trailer is pretty useless):
http://www.ohlalamag.com/.a/6a00e54f...5d5e1970c-80wi
American artist Don Bachardy still lives in Isherwood's California home and also had a cameo part in the movie as a fellow professor.
He is a portrait artist however and I am sure your reference to nude young guys in pools in California means the English artist David Hockney.