The versatility player, who signed under Darren Kelly has played in a number of roles for the Seadogs.
He leaves to join National League North side Farsley Celtic.
Boro boss, Jonathan Greening said: "It is a blow to lose Lewis, but he has a desire to play at a higher level and the move is on his doorstep.
"We all wish Lewis good luck at his new club and he leaves on good terms."
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