I would say it's dirt. I went through the same thing with a 5D Classic back in October. Went to a local camera repair shop, they couldn't clean it, and I couldn't do it as well, wasting $$ on cleaning fluid and such. Sent it to Canon, they cleaned it just fine.
One question: if he tries cleaning himself, do the dust particles move around, or are they in the same areas? For me, the particles were always there, but when I tried cleaning they moved around, they weren't always in the same place.
Hope that question made sense...