If a guy was exposed to some chemicals many years before conceiving, is it possible in the future, he may have kids with new genetic defects?

Not fully known: There's not enough science to know for sure. There is evidence that relatively recent chemical exposures before fathering a child increases risk of birth defects, though exactly which chemicals cause this isn't fully known- see http://www.livescience.com/21664-dad-job-birth-defects.html Some chemicals, such as pesticides and dioxins, can persist in our bodies for many years,so this seems possible.