Teach him these as you teach him the names of other body parts. All young children handle their genitals. Normal children like to explore all parts of their bodies. When they handle their genitals and find that this feels good, they may rub them. They may masturbate when they feel bored or upset.
Some masturbation does no harm.
It is normal and it is best to ignore it. If you try to stop it and tell your children that it is bad, they may feel that they are naughty, or that sex or sexual feelings are bad. Sometimes, when young children play together, they push, hit, slap, or bite. Biting and forceful hitting must be stopped right away. Most children bite and hit when they are feeling angry.