I would think your leopard3 should be good enough since you had already said you didn't want to buy a new disc. I could be wrong, but since the leopard is another disc that is always mentioned as the starter/trainer learn to throw flat disc.
Are you saying for him to throw an understable disc and correct his form until he throws it level? To me it sounds like he is rolling his wrist or releasing at a strong anny so that's why I suggested a putter and practice releasing on a strong hyzer then eventually releasing it flat. ?