I have a note from my doctor stating I need to work from home 5 days a week. I was hired under the American's with Disabilities Act and receive a Disability check from the VA every month. Now my manager is asking for more medical documentation at "back up" the note from the doctor and is requesting a report from my primary care doctor defining my disability. Is this right? Can he do this?

Yes, they have a right to know full information about the disability. They are only required to do those accomidations that are within reason and affordable. So they may get a full and complete doctors records to see if and what accomidations are really needed.

Work from home ? Why.
What disability issue is there getting to work ? What can they do at work, to make your work station usable the same as home, a chair, additional rests ?
I may find work from home requirement questionalbe for almost any disability.