The problem with congestion is that the underlying cause is viral so antibiotics really don't help THAT part of it. That part must work its way out of the body, fought off by the immune system.
If you are still seeing congestion and it is clear, you can keep giving the antibiotic and treat the symptoms. To help clear congestion from the nose, you can use pediatric nasal saline drops (not adult) and you can use a humidifier.
You will have to see the vet for a thorough exam of the nasal passages to see if he has a blockage or are that needs to be widened.
Let me know if you still have questions.