mtoto mwingine wa will smith...TREY SMITH....the only not famous smith mchek hapa(thanks tmz)

Will Smith's firstborn son co-starred with him in the "Just the Two of Us" music video -- but we hadn't seen much of Trey Smith since then ... until our camera guy bumped into him in the Valley.

Trey's not nearly as famous as his dad -- or his little siblings -- but check him out ... still an awesome guy.