The new DAN protocol says that if the gut isn't treated before or at the same time as chelation, then if your child has Candida, like many of the kids, then the chelation products will flare it up and it may even "mask" his progress, in a sense.
What strikes me about your son's nasal congestion is that a friend of mine has had sinus problems for two years now. And she has tried everything. I've been reading up on Candida and all the negative effects it can have with autism (destructive behavior, aggression, hyperactivity, etc.) and I came across something about sinuses and Candida. I sent it to her and she is treating herself for it now because it sounded so much like her situation.