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Repeating phrases and words is also known as echolalia. It can be normal in children you son's age and in at least 1/3 of children with this condition it resolves on its own by 5 1/2 years. The fact that he is very verbal and it only occurs at home are good signs that it is more behavioral in nature than due to a neurologic problem. It is similar to a tic, which can be seen in healthy kids your son's age as well (also known as benign tic disorder of childhood). I would try not to make much of it as bringing attention to it can reinforce the behavior. You can ask your pediatrician about getting a referral to a speech therapist if you are very concerned about it to get a more through evaluation done.
Please let me know if there is anything more I can answer for you.