Stepson doesn't wash his hands after using toilet. Am I being overly worried?

Try to teach. The main reason for washing is for other people's safety and hygiene. Our urine is sterile most of the time. Colon is loaded with over 1000 trillion bacteria in it so impossible to keep everything sterile. Studies show that family members eventually acquire each other's bacterial flora after living together so there is some spread naturally. Washing our hands keeps others safe. Good luck on boys.