The 20-year-old midfielder will hope to gain further first-team experience under manager Kevin Bond at Roots Hall, after impressing on loan at another League One club, Rochdale, last season.
The Academy product enjoyed a successful stint with The Dale in the second half of last term, scoring four goals in fourteen appearances.
Hamilton is the sixth player to make a temporary move away from United during the window, which closes on 2 September for League One and League Two clubs. The process enables our young players to benefit from facing senior opposition on a regular basis.
Good luck to Ethan for his loan spell at Southend.