The issue is that you have your character losing the ability to speak as a young adult. There has to be some kind of trauma (physical or emotional) for that to happen. The neurological pathways for language are formed starting in infancy (they really get going around 9-10 months) and continue for several years. If your character knows how to talk at 20, he's not going to abruptly lose the capacity just because he didn't use it for several years. It's not like walking where you lose muscle tone.
(Also, I don't know about you, but I talk to myself all the time even when there's no one else around. I laugh out loud at things I read, I call the cat, I sing along with music, I talk back to the TV, I yell obscenities if I hurt myself, I mutter if I'm looking for something. Being in isolation for several years does not by itself mean I won't speak for several years.)
Now, there's a difference between losing the ability to speak (he wants to, but can't) and losing the desire to speak (he can, but doesn't want to).
It might be more interesting if the reason for your character's muteness is a mystery which the other characters (and the reader) have to figure out. There either is some trauma which makes him afraid to speak, or he took some kind of vow of silence so he's choosing not to speak. That could be political, religious, cultural, or just personal persnicketiness.