It could be fungal sinusitis. A CT on the sinuses will probably help. He may need the antibiotics for three weeks instead of the usual 10 days. Is he in daycare? You need to check out where he is picking up the infection/fungus as well...maybe the daycare isn't up to par or there is another child with whom he plays with this same problem.

I don't want to scare you, but I was wondering, with the strong smell of iron, if it is actually old blood, which could be coming from either his sinuses or farther up inside his head? Did they do a test on the mucus to see what it was comprised of? They should do a lab test on it to best determine this.

might be a bit late on the reply but my son had the exact same symptoms when he was about 2. turns out that he had swallowed a penny and it got stuck in his esophagus. He got a bad sinus infection along with blood tinged mucous and the smell along with the color was awful. im interested in knowing what they found. keep us updated