Sounds like he's ill wallsie. As rowanwitch says, budgies are highly social birds and they need a companion. We have two budgies in an indoor aviary who spend all day playing and chattering to each other. They also have various toys and several large mirrors in front of which they spend a lot of time feeding, playing and chattering to their reflections. They also have wild birds visit them at the open window and they interact with those (and mimic them!) Our birds also twitter and chatter to the washing machine, TV etc.
I think your budgie is now being affected by isolation. As budgies are flock birds and highly social, isolation eventually causes them severe mental and behavioral problems. Look what happens with most animals are locked up in zoo cages with little company and no space. They develop repetitive behaviour, self-mutilate, continually chatter or go quiet etc. It's the same with people locked up in jails or isolated in society. The reason single birds mimic or 'talk' to human owners is because they are so desperately lonely they talk to humans as surrogate companions.
It might be an idea to find your bird a companion, preferably a hen. Introduce them in separate cages over a week or so and then put them in the same cage. I think you'll see a world of difference in your budgie. Also, do you keep the bird in a small cage? Those form tiny prisons in which the bird can hardly move. Birds need as much space as possible to fly and excercise. We have an indoor aviary which takes up a corner of the lounge. They are expensive to buy but fairly cheap to build yourself from items from B&Q etc.
I hope your bird gets better soon.