It's good to know that he trusted you with this information and he knows that the people are imaginary. This could be a transient psychotic attack but a visit to the psychiatrist will reveal if there is anything else which needs medical attention.

I understand your concern towards your son. He seems to be suffering from psychotic symptoms. However more clarity is needed regarding his problem to decide on severity of the condition. Don't worry it is not his fault, it is the hormonal and chemical fluctuations in brain as a result of stress which is taking a toll on his mental health. It is easily manageable with appropriate treatment, so that dysfunction due to the same can be reduced and his quality of life will improve. Some counselling sessions along with medical help might be useful. I would also be more than happy to help you with that if you are looking for online assistance. Discussions will be held confidential. You can directly consult with me through Practo as needed. Online chat consultation is also available.