It's entirely possible, and quite common actually, for one species to have ick in a tank and the rest of the bunch remain unaffected.
Doesn't mean it will be that way with you, but it's very possible.
Does the end of his tail look sort of like drapes hanging down onto the floor? Kinda wadded up, fluffy sort of? That's what all my betta's tails look like and I have yet to figure out why. I have tried everything to get rid of it. Their dorsal fins also look like the curling ribbon that is on top of birthday presents - haven't figured out how to fix that either. I've medicated with several different types of finrot meds, but nothing has fixed it. They're (for the most part) otherwise healthy guys.
When he rubs himself on the plants, is it just his fins dragging across them, or does he rub his sides on the plants, almost like he's scratching himself??