EXCLUSIVE

Nick Young forgot all about his former high school basketball coach when he made it big in the NBA ... at least that's what the guy's saying in a new lawsuit.

Andre Chevalier filed a $700,000 breach of contract suit against the Lakers star. Chevalier says he signed on to be Nick's business manager and consultant as he was entering the NBA draft in 2007.

Chevalier says his compensation would be 2% of Nick's endorsement dough and his NBA salary. He wasn't totally getting stiffed ... he admits he's been getting a cut of the money Nick pays his agent -- but says in 2015, he realized the 2% from Nick was missing.