I too believe Boromir needs some improvement as he's gone without any change for a long time, while in the meantime new factions have been created with Tank-heroes that establish this category better. Think of Thranduil with his stances, Arcane Shield, and Shield Wall, or Karsh with his life steal and Wight-support abilities. Compared to them Boromir (and other aspects of Gondor) feel left behind, but there are plenty of ways to improve his performance while remaining unique.
This is a feeling I generally have with the Edain Mod: it seems that Gondor is the most neutral faction that creates the degree of uniqueness towards other factions. This is not meant as criticism saying that Gondor is not beautifully designed, but the faction deserves some upgrades to keep up with the overall quality of the mod.
Regarding Boromir, it seems we can draw from two different sources to inspire his abilities: his time as a Captain of Gondor, and his time as part of the Fellowship. It would be interesting to allow him to reflect this difference in more than his aesthetic change at level 5, by including a toggle weapon system such as the Horn of Gondor or his shield next to his sword. The difference would lie on aspects such as a focus on unit support when equipped with his Horn, and a focus on tankiness when equipped with his shield.
EDIT: I have come up with a concept that incorporates the suggestion of a switch weapon ability and have come up with different identities for the Horn or the shield.
Level 1: Shield/Horn toggle
When equipped with the shield Boromir gains more armour (15-25%). When equipped with the Horn of Gondor Boromir hits two targets with every attack. The passive ability Last Stand is incorporated here as well: when Boromir reaches (very) critical health, his attacks deal knockback damage.
Level 3: Horn of Gondor (when equipped with Horn)
Boromir blows the Horn of Gondor, stunning nearby enemies with fear.
Level 3: Valour (when equipped with Shield)
Boromir shows his valour for his people, encouraging soldiers to fight with more skill. Nearby friendly soldiers gain increased amounts of experience for every slain enemy, while he stops gaining experience for a short duration.
Level 5: Heir/Captain of Gondor
Boromir gains his heavy armour, and passively gives an armour bonus to nearby friendly units.
Level 7: Brave Resistance (when equipped with shield)
Boromir gets ready for a long fight. His armour is increased drastically and he becomes resistant to knockback for a medium duration.
Level 7: Banner of the Steward (when equipped with Horn)
Boromir places a banner of the Steward in order to inspire units when he is not nearby. Boromir's leadership (armour increase and increased experience gain to nearby units) is split, but does not stack when he is nearby and is limited to 1 on the battlefield.
Level 10: Veterans of Osgiliath
Boromir summons four battalions of fully upgraded veterans from Osgiliath to help him in the fight.
The aim of this suggestion is to provide two identities for Boromir: one that emphasises his strenghts as a lone tank, and one that emphasis his abilities as a Captain of the Men of Gondor, supporting units in the fight. In his Ring-form the 'Banner of the Steward' ability can fill up the fifth ability spot that is currently empty: it will allow Boromir to split his excellent leadership on more places on the map, allowing the player to create an almost impregnable line of defence.
I will say beforehand that the abilities need some tweaks but I believe this creates an interesting approach to the magnificent Captain of Gondor.