A couple of Feat/PrC ability ideas...
No Weak Links
When taking the Aid Another action, you may do so as a move equivelent action.
The Chain Has No Links
You may aid another as a free action.
(This is to represent the "Bussaw " tactic used by legionaires, attacking the foe to their side to bypass the shield.)
Disabling Throw
You may use a Pilum (Javalin) to attack an opponents shield. If sucessful, they must make a reflex save (DC?). Failure indicates that the shield is rendered useless for defense. Sucess indicates that the shield's AC is retained. In either case the Pilum is lodged in the shield, adding an armor check penalty of 5 in addition to it's other effects. It may be removed as a full round action. Tower sheilds are not affected by the loss of AC, but still gain the additional armor check penalties.
Greater Disabling throw
When using the Pilum in an attack and you miss you target by the amount of protection of his shield, you may make a free Disabling Throw attack, as per the feat.
None Shall Pass
When in shield formation, the DC for Tumbling is increased by 5. Tumbling is also not automatic, any attack of opprotunity that hits and does damage to the Tumbler stops him in that adjacent square. Bull Rush attacks are also far more difficult. Any ally adjacent to the target of the Bull Rush adds 1 to his roll to beat the attack.
(I think this fits best as a first level PrC ability).
As a character, you might look into some ladership feats, or something like the Warmaster PrC. Most of the stuff so far would make him shine in a group but not as an individual player.