it looks like quite a bit of hit to the head, cause bleeding from nose is not common on falling backwards on the head.. it usually indicates an injury
to cribriform plate (roof of nose) or sphenoid bone
.. although its unusual to injure these areas by hitting back of the head.. it is advisable to take him to a doctor immediately and get a check up done including scan of the head which can detect any on going bleed or injury to internal structures.. and necessary measures can be taken immediately..
although the bump on the head may cause a bit of pain, occasionally irritability in child or sudden crying episodes may indicate raised intra cranial pressure due to bleed within.. even if child is comfortable now it is better to get the child examined by a pediatrician
and rule out any internal trauma as soon as possible..