"His diet is not that good, he likes eating pizza, rice with chicken."

If he has a candida problem any starchy foods wouldn't be good. Pizza is bad since it often has plenty of sugar, yeast, and all the carbs in the bread.

No detectable blood mercury? A blood test usually doesn't mean much, although I think there should be at least some detectable in mercury in his blood if he has a mercury problem. Who knows though. It is hard to test for mercury. Some doctors do challenge test and give a relatively large DMSA then collect the urine for 24 hours and see how much mercury is in the urine. I guess they collect urine before as well for a comparison. Some people feel very sick after a challenge test.

I wonder if he might be toxic from some other toxic metals besides mercury, or if he has mineral deficiencies.