I think he will still be up for a challenge to improve his grades, so I would look at it more as a goal to aim for.

I remember when maths was split up at my school, and dumb maths was called vegie maths (as anything considered fun or not challenging was a veg out subject) and there really wasn't any incentive to improve. It was considered by staff and students alike that once you were in those classes, you didn't want to do maths and therefore no effort was made on either part. Just enough to get you through it. Students would boast about being in vegie maths, hardly the environment to make you want to do well.

But I think if he really does want to improve to get the higher grades to get his phone fixed, then you might need to help him set up mini goals to let him see his achievements for himself. It will also let you see how dedicated he really is. If he can't reach mini goals, or at least doesn't try to, then you will know that he really isn't that interested.