It's in "The Steward and the King", not long before the coronation of Aragorn.
The description is: "bright ardent like snow in the sun, bearing no charge or device"
A small error there; the full description is:
and upon the White Tower of the citadel the standard of the Stewards, bright argent like snow in the sun, bearing no charge nor device, was raised over Gondor for the last time.
Argent being the heraldic term for metallic silver, and also white, coming from the Latin Argentum (silver, or white metal.)
The unanswered question is whether he would wear the heraldry of the Steward (an office he did not hold) or that of Gondor (a Kingdom that he was representing).
Faramir would have been a good point of comparison, were he not dressed in camouflage when we see him.
Another point of comparison we have is the guards of the citadel, but unless Boromir is counted among their number then it only helps us to know how he isn't dressed:
The Guards of the gate were robed in black, and their helms were of strange shape, high-crowned, with long cheek-guards close-fitting to the face, and above the cheek-guards were set the white wings of sea-birds; but the helms gleamed with a flame of silver, for they were indeed wrought of mithril, heirlooms from the glory of old days. Upon the black surcoats were embroidered in white a tree blossoming like snow beneath a silver crown and many-pointed stars. This was the livery of the heirs of Elendil, and none wore it now in all Gondor, save the Guards of the Citadel before the Court of the Fountain where the White Tree once had grown.
emphasis added.
We also have description of the men from the Outlands, many of whom are dressed in uniforms of sorts from their regions, but few are bearing heraldic tokens. The most noteworthy is Imrahil:
Prince of Dol Amroth, kinsman of the Lord, with gilded banners bearing his token of the Ship and the Silver Swan, and a company of knights in full harness riding grey horses; and behind them seven hundreds of men at arms, tall as lords, grey-eyed, dark-haired, singing as they came.
This seems to indicate that most of the heraldic devices are those of the respective lord, rather than personal devices.
So, it seems we are left with little description (a shield) in Book 2 Chapter 3, his shield is not mentioned in Book 3, is only mentioned in the lament for Boromir in Book 4 (his cloven shield), and then we are given examples of what it probably isn't from Books 5 & 6.
The banners in Book 6 are representative of the office of Steward, which being hereditary makes it like a familial identifier, but at the same time legally is likely to apply only to the current office holder.
I don't see how anyone can make a definitive statement regarding what Boromir's shield should look like, but this from Book 4 seems to indicate that Frodo didn't find his shield to be remarkable:
A shadow of sorrow passed over his face. `Do you remember aught of special mark that the Lord Boromir bore with him among his gear?'
Frodo thought for a moment, fearing some further trap, and wondering how this debate would turn in the end. He had hardly saved the Ring from the proud grasp of Boromir, and how he would fare now among so many men, warlike and strong, he did not know. Yet he felt in his heart that Faramir, though he was much like his brother in looks, was a man less self-regarding, both sterner and wiser. 'I remember that Boromir bore a horn,' he said at last.
So, with a horn being the special mark, a plain shield is completely supportable.
My investigation leads me to think that the Gondorians might not have relied on their shields for heraldry, but instead relied on banners, with the possible exception of the Guards of the Citadel. Pippin's shield
is described as having a device of Gondor.
The only other shields that are noted as having devices are those of the Rohirrim, Saruman's forces, and the Orcs of Mordor.