It may be a part of his symptoms. The symptoms tend to be different for everyone. I have BP1 and I keep my house really clean.

It may be that he is too depressed to have the motivation to clean or maybe it doesn't bother him. How old is he? I have walked into a lot of 20-something guys apartments that are about exactly how you describe, and most of them don't have any kind of mental health problem. In fact I was at a good friends house last night and he has about 20 pizza boxes stacked on the floor (leaning tower of pizza). It disgusted me, but he seemed quite proud of it.

Maybe ask your son if he would like some help cleaning up. It may be that it just doesn't bother him.