Definitely change his water source, but I DEFINITELY wouldn't give him bottled water! Multiple studies have shown over 50% of bottled waters contained levels of e coli & other bacteria too high for safe consumption. There is NO regulation of bottled water, whereas there is for tap. Definitely have your pipes checked, but if they're fine, just use a filter. Reverse osmosis is best.
For now, I would buy him gallons of Spring Water - the healthiest anyone can have.
I know how to detox for adults, but i don't know for children. There are products available at natural medical suppliers that you can take internally (herbal) that bind to the heavy metal & excrete it out of your system.
I would contact Dr. Tel-Oren immediately - he founded the Ecopolitan all-raw organic restaurant on Hennepin. You can read more about him here: http://www.ecopolitan.com/doctor-t-cv-full
Trust me, I'm a skeptical person & I've gone tried seeing many different holistic practitioners. He's the smartest person I've ever met, and most of the time, he's steering me AWAY from products & telling me which things are scams.
The easiest thing I can think of (but you should still research effect on small child) is just giving him an epsom salt bath. Epsom salts are very cheap, available at most food stores (I know they have at Wedge, Seward & North Country Coop). Pour into a warm bath, and they will draw the toxins out of the body.
And LOTS OF WATER!!! I would avoid all other liquids (except breastmilk). Water is the best purifier to flush toxins.
Good luck to all of you. Peace.