Please don't give up. I think that you need to add ALA with the DMSA. (also called lipioc acid) It can be found in the health food store. Start with low doses. (25 mg...divide if you can only find 50 or 100mg)
Many people that are chelating don't see changes in the "autism" part of their children until they add the ALA. The DMSA will chelate the mercury out of your child's GI tract. The ALA is the only thing that I think is known that can cross the blood-brain barrier and release the mercury from the brain. When ALL of the mercury is freed from the brain, your child will be NOTABLY better. The behavioral problems can be from the GI tract, too. But the speech delays, visual perceptual problems, most of the sensory problems, I think are from the mercury in the brain.
Dr. Andy cutler from yahoo groups Autism-mercury groups has a lot of experience helping people chelate. He believes that when All of the mercury is gone, the brain CAN AND DOES heal. This does not mean that you will not see results with each round. Each brain cell affected may have mercury clung to it. When you get the bottom mercury off the cell, then that brain cell can heal. So if you free some of the brain cells, they can start healing while they work on others.
Don't give up prematurely. Chelation is the hardest thing that I have done. But it has shown the best results of any intervention. (my son is 7, almost 8)