Getting a second opinion from the new avian vet is your best option in my opinion so that he can be assessed. Although this is not parrotlet specific, it talks about issues with parrots are they age.
You are very lucky that your bird is able to successfully live in an indoor aviary currently. Most parrotlet species are not kept in aviaries due to their aggressive personalities with both their own kind and other bird species.
It sounds like you are giving him a wonderful life and he is cute in the photo