Three tips:
1- Always have someone in melee contact (preferably an elemental of some other summoned pet). This prevents him (and all dragons) from using his breath weapon. I think Mordenkain had dragons in mind when he developed his sword spell.
2- Cast lower resistance (5th level mage spell) 3-5 times. Having multiple mages helps here. I usually do my dragon slaying almost entirely with magic, the fighters are just there as heroic decoys. About 100 magic missiles ought to do the trick.
3- The elementals can’t touch him because he normally cast stoneskin is addition to his AC of –12. This calls for a breach. Casting breach can change the melee dynamic with any dragon in a real hurry.
There is no shame in coming back when you are stronger.
-Cayle