Flak / Cluster Bomb
Either have this attached to his current Airstrike ability or give an additional nade. The reason for this ability would be to give him some usage while inside/underground.
Option 1) Cluster bomb would be used as his primary ability, the Airstrike. Maps would be filled with pre-determined volume brushes to ensure correct usage. When inside with little chance of damage being done via an Airstrike the locator nade explodes into multiple nades that detonate 2 seconds later. I would have the mini nades visible to the players and spread randomly, kinda like the alternate fire on the Flak Cannon on UT.
Option 2) Flak Nade/s would be assigned to the G key by default and act like a standard nade. on detonation a AoE of damage starts and lasts for 3 seconds. Damage would be around 30 per second for players within the radius. You could even have damage calculated based on how close to the centre a player is. This ability would use a flak style effect for visualisation. Possibly 2x damage to mechanical objects.