You should seek an immediate medical exam for your younger brother. Suppose he is suffering from internal injuries from getting thrown down the stairs? It could lead to later complications that result in his disability or even death. Who knows what other injuries the boy could have sustained out of your sight from similar or greater abuse? Remember that often homeowner's insurance will cover these types of injuries, if you're concerned about lack of medical insurance.
Seems to me anyone who throws their son down a flight of stairs is mentally ill and a danger to themself and others. Have the boy examined by trained medical experts. It might get your father into serious trouble, but save the boy if he is internally injured, perhaps suffering from ruptured internal organs that may require surgery?
It horrifies me to think of a child that age, still in their developmental stages, suffering internal injuries that could heal improperly leading to serious complications, disabilities, even sudden death years later. I have known some to turn the matter over the the local body of elders, but IMO you should consider informing the secular authorities (i.e., Social Services) to effect a tangible resolution that will protect your younger brother from future abuse. Just expressing my concerns!
Derrick
To see a World in a Grain of Sand
And a Heaven in a Wild Flower,
Hold Infinity in the palm of your hand
And Eternity in an hour.
-- William Blake (Auguries of Innocence)