You can flat out say something but you may come off rude.
You could put out hand sanitizer all over your house.
Say something like would you mind washing your hands or using sanitizer my little guy catche's everything so easily but you might get some crazy looks or offend people.
From now on out your son will be meeting people everywhere he goes.Local Park, Daycare,Preschool, Library time, Cousins, School, Junior High etc.. and I'm gonna bet most people don't take handwashing as seriously as you do.
My daughter is in kindergarten and we are very good at hand washing etc. She has missed so much school this year due to illness I am now being "watched" by the school and need a doctor's note everytime she misses. Totally sucks and makes me mad. I NEVER send my child to school sick but obviously alot of other inconsiderate people do.
And really what can one do about it. Not much we can only control what we do and what our kids do.