Children, when full of the Holy Spirit, are a source of joy.  When full of evil, they are a great bitterness and cause of pain.
This is true whether the evil is in their behavior or, as here, in demon possession.  You can hear the agony in the father's cry.  Jesus has come down from the Mount of Transfiguration only to find the failure of his disciples.  He quickly discovers the source of difficulty:  a misplaced "if."
The misplaced "if"
"If you can?"  The words come from Jesus as a stinging rebuke to all who hear.  He is God the omnipotent;  there is no "if" with him.  The "if" is with us.  For all things are possible--if we will believe.
So many of us don't believe that, especially about ourselves.  We think, "Oh, I'm such a sinner God could never make anything special out of me."  That very thought is blasphemy, for it denies both the power and the goodness of God.  So then, how do we achieve such faith?
Do you lack faith?  Do what the father does here.  Go to God and ask;  tell him you have a little and want more.  He wants you to believe;  he will be pleased to grant your request.  But do recall that no one ever reached perfection in a single bound.  It will take some time for this faith to increase;  for God has chosen the method of prayer as the method for increased faith.
It is interesting here that Jesus permits the demon one last moment of control over the boy.  Perhaps it is to demonstrate to all just how wicked this demon is, to torment a small boy this way.  It also serves the purpose of squeezing more faith out of his father.
Christ gives the demon two commands:

• Get out.
  
• And stay out.  We need to remember that a momentary victory over evil always carries with it the possibility that the evil will return.  We should look to our spiritual defenses after a spiritual victory.