"'"If you can"?' said Jesus. 'Everything is possible for one who believes.'" NIV translation
The man did not understand who he was talking to just as the disciples did not fully understand. Jesus explained that it was not a matter of whether He could heal the boy but a matter of whether the man had the faith to ask.
"Immediately the boy's father exclaimed, 'I do believe; help me overcome my unbelief!'" NIV translation
The man had the faith to bring the boy to Jesus but needed the help of Jesus to overcome his doubts about the healing of the boy. We all have some small part of us that limits what God can or will do and it is only through Him that those limits (in our minds) can be lifted. That is a big part of walking and growing in our faith.
"When Jesus saw that a crowd was running to the scene, he rebuked the impure spirit. 'You deaf and mute spirit,' he said, 'I command you, come out of him and never enter him again.' The spirit shrieked, convulsed him violently and came out. The boy looked so much like a corpse that many said, 'He's dead.' But Jesus took him by the hand and lifted him to his feet, and he stood up." NIV translation
As Jesus saw a crowd forming, He quickly cast out the spirit. Jesus was trying to proceed with His Father's business and did not want any further public notice. The religious leaders were already very jealous and He knew that it was not the time for their wrath to come.
The unclean spirit immediately left but the boy was left in such a state that those around him thought that he was dead.
"After Jesus had gone indoors, his disciples asked him privately, 'Why couldn't we drive it out?' He replied, 'This kind can come out only by prayer.'" NIV translation
The disciples had forgotten the source of their power and they had neglected prayer. We, too, must remember that our power comes through our relationship with Him and a healthy relationship will include conversation with God (prayer).