According to my father, my German great-great uncle (my great-grandmother's brother) did some time with Hitler. He may not have been his cell mate, but (supposedly) was in the same prison as Hitler. I've tried searching the Internet via Google for information, but this kind of information is likely to be found in a book (or in the head of a historian). The last name of this relative is Willi.
Landsberg Prison has a capacity of 800 inmates. It is not unlikely that your relative was held in the prison at the same time as Hitler. Considering that your relative was probably not a prominent inmate, I doubt that there is any information on him in either books or the heads of historians. Instead you will want to check the Bavarian State Archives for records from the prison. I do not think such lists are likely to be digitized however.