troubled minpin

i have a very troubled minpin when he gets out he refuses to come when hes called...hes been to obedience school and i beleive it was a waist of money...he is a great dog when hes in the house but when he gets out hes in a totaly diffrent state of mind....how do i chanhe this behavior

You work with him outside. In the beginning with as few distractions as possible. Start out at short distances and gradually increase the distance as he gets proficient at the one you are working on. Start at about 6'. Get a long line to use with him.