Yes, it is true that Lord Hanuman have a son. Name of his son is "Makardhwaja", which belongs to Samundra (Sea) and Patal.
The story about Lord Hanuman and Makardhwaja fight is mentioned in Adbhut Ramayan. According this Story:
When King Ravana was sure about his defeat in The war with Lord Rama,
He approached Ahiraavan to help him. Ahiraavan was the king of Patal
Lok, and Makardhwaja is the Protector of Patal Lok.
When, Ahiravana tricked and Kidnapped Shri Rama and Laxman and took
Rama and Lakshmana to Patala. Lord Hanuman followed them to their
rescue, where at the gate of Patala Lok he met his Son "Makardhwaja"
when Makardhwaja introduced himself as the son of Hanuman and told the
story about his birth, Lord Hanuman Accepted him as his Son.
Then Sri Hanuman told him to save Sri Rama and Laxman, But
"Makardhwaja" said that Ahiravana is the Master of Patallok and as you
are doing your duty i am also serving my master to protect the patal
s why i cant allow you to enter in Patallok, then to save the life of Sri Rama and Laxman, Lord Hanuman fought and defeated his son "Makardhwaja".
You can watch this Video in Hindi which describes Panchmukhi Hanuman Story, the Complete Story and Birth Details of "Makardhwaja" and his Mother.