At 6 months-old it could possibly be that he is starting his first molt, which is the largest molt of his life because they shed their baby-feathers and grow-in their adult plumage...However, 6 months is a bit young to start his molt, though it is still possible, but the problem is that he still shouldn't have any "bare" spots where he has no feathers, that's not how they molt...
Can you take a photo of the "bare-patch" on his shoulder and post it here so we can see what is going on? Because without seeing it we can't tell you whether or not it's a problem...I will say that often times they do pluck the top of their shoulders, so that's why we need a photo to be able to tell you much...But you do want to nip it in the butt if he is plucking already for some reason...
***Just to make sure, is he housed with any other birds in the same cage, or do you have any other birds that he is with outside of their cages? And did this "bare-patch" just suddenly appear out of nowhere, or did you see it develop over-time?