I think I have only heard Asher say the word "yes" once. He usually prefers "okay" (like it was your idea, not his. ex: "Momma, Yodder Pops?" "What? You want an Otter Pop?" in a sweet, happy voice that sounds a bit surprised "okay!") or "sure" (pronounced "tsure", used in the same way as "okay" but more careful sounding) and, occasionally, "yep."
I'm not bugged by this. It does create some confusion when we have to tell him no, but I'll take it over a blank stare, grunting, or pointing and screaming any day.
(Insert cute picture of Asher here. My pictures still ain't workin'.)