48hours ago your son fell on his back....I am assuming that he has no problem moving his legs and that has had no vomiting or headache.
Almost certainly, this is no big deal and i would take ibuprofen 400mgms up to every four hours with a hot bath and this should settle over the next week.
Taking a pessimistic view, he may have cracked or even broken the transverse process ( part of vertebra) but even so ....no big deal and the treatment is the same.
monitor the situation for the next 3-4 days.