You’re not going to get pregnant from your step-son masturbating and not washing his hands. Sperm have to be released in the vagina and be able to “swim” upstream to your eggs.
You don’t say how old this child is, but it’s pretty normal for kids to masturbate. Especially, when they get into (or close to) teenage years, (putting it very simply) they have raging hormones and begin discovering their own body - aka masturbation. Although you can’t get sperm from things he’s touched, he should be reminded to wash his hands on a regular basis and not touch things that others will touch if he’s not washed. It has nothing to do with you being able to get pregnant (you can’t); it’s about not passing germs around. Just like we’d teach our children to wash their hands after going to the bathroom, this child should be taught to wash his hands after masturbating.