It's clearly stress, even if the vet said everything's physically fine. Greys are hypersensitive to their environment and routine - two months is exactly the timeframe where they "shut down" when something's bothered them. Speech is the first thing that disappears in them when they're anxious, even before other visible symptoms show up.
Beyond the obvious changes (moving, new roommate), also look at the less obvious stuff: a window that's closed now, less natural light, a piece of furniture moved, disrupted sleep schedules, even someone yelling more than they used to in the house. Greys freak out over details that seem totally insignificant to you. If none of that fits, ask yourself whether you're interacting with him less - sometimes you don't realize you've changed your habits, but a parrot picks up on it right away.
The good news is that if it's stress, it usually comes back. Try creating an ultra stable and predictable routine for a month or two, lots of quiet time with him, and above all don't pressure him to talk. He'll start speaking again once he feels safe again.