If so, you can plan a reward/punishment system and write it down on paper and sign a contract with him, I am serious...this is called negotiation and involvement, taught by social workers.
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If he passes and average grade on major subjects...a grade you are both satisfied with, then give him a reward. If not, give him the punishment.
Of course, if you set the goal so high that he can never achieve it, he will not even bother to try.