Lead is heavy and falls out of the air quickly. Keep window wells clean. Damp mop floors frequently. The dirt and dust you track in from near those streets will contain a lot of lead dust. Wash his hands and toys frequently.
Nutrition is also important.
Make sure his diet is high in iron and calium. They block the absorbtion of lead that is ingested. Also, check your water. Pipes may be lead or be soldered with lead solder. Lead poisoning is cumulative and you want to reduce all possible exposures.