When chemical (not sure what) induce mutation in spermatogonial stem cell, what's the increase in risk of having de novo genetic defect child?

Not measurable: This has been answered, Jim. Every cell in your body probably bears a somatic mutation or two. Mutating genes is nature's way, a fact of life, vital to the overall health of species. Part of growing up -- which you are doing now -- is accepting life's risks in stride. It's good you care about your children's health but this risk is both tiny and unquantifiable

Be practical: We all carry a few recessive genes that could be a problem.That's why we don't have kids with our sisters. Any chance effect during spermatogenesis is limited by the process of reproduction. It is the healthiest sperm that get to the egg first, so any nasty mutation that would impair the sperm would make it a poor swimmer. Men get this natural pass to some of their exposures not shared by women.