inclusion?
I'm picturing more of just a metal cap than a huge, heavy helmet with visor or a great helm. He's obviuosly travelling light, and probably kept it in a pack or hanging somewhere in his gear, hence why it isn't mentioned until his end. "His helm they set beside him" and that's it. Very strange, though, the sudden appearence of it.
For edification:
sutton hoo and a
norman helm, the closest I could come to what I think ME helms would be like. I think Boromir's would have been nicer, less plain, and designed to be worn without a mail coif, because it's fairly certain he had no mail. Elendil wore a fancy one in the film, but I don't picture something that stylish. The elvish helms in the film were obviously based on ancient greece, which gave them a nice, different look. Anyway...