Levi Johnston -- I Want 50/50 Custody of Tripp Palin

Bristol Palin's baby daddy Levi Johnston is tired of getting squeezed out of their son's life -- TMZ has learned, he just filed court docs to obtain 50/50 custody of 4-year-old Tripp Palin.

In the docs, Levi says he wants Tripp to be in his life as much as Bristol's because it's in their son's best interest. Bristol currently has full custody of the child, but now Levi wants Tripp on alternating weeks. (They both live in Alaska.)

Meanwhile, Bristol's attorney is insinuating it's simply a move to reduce Levi's child support payments, saying Levi owed roughly $66,000 in back child support as of October -- and now he wants to pay less.