The time traveller has been driven insane by visiting other times
The Time Traveller has returned to his own time, but can barely put a sentence together. It seems that he is struggling with his words and his thoughts and cannot tell if all of his adventures really happened or if he might have just imagined them. His travel is confirmed though, when they walk through the house and find the actual time machine.
The child is the father to the man means that we will find the basic personality of the adult nascent in the child. Is the child honest, extroverted, sly, cautious, thoughtful? Driven to succeed? Driven for approval? Fatalistic? You will likely find that when he is an adult he will embody much the same character.