Gentle pushing on your belly as it gets bigger is fine, and this is what most kids do. Hard jabs, kicks, or punches can be dangerous, particularly as you get farther along in your pregnancy. Trauma to the uterus in any form (a hard punch or kick to the uterus, a fall directly onto your abdomen, a car accident) can cause something called a placental abruption. This is a condition where the placenta pulls away from the wall of the uterus. In a mild case, there may be some vaginal bleeding and/or contractions with no consequence for the baby. But, in a severe case, a large portion of the placenta pulls away and could cause problems for the baby.
Teach your son how to be gentle with your belly by teaching him to "pet" or "stroke" stuffed animals or real pets (if you have them). Or have your partner "pet" or "stroke" your belly gently to set an example. If your son continues to be rough or if you are ever kicked or punched (b your child or anyone else) while pregnant, contact your doctor for advice.