Hello
Since when is started having pain.
Does the pain goes down in his legs.
Has he done any investigations
Next Steps
ideally anterolisthesis means forward slip of one vertebra over other. but there r grades of it and it's important to decided the treatment on basis of the symptoms and severity of grading.
Health Tips
I would like to see the x-ray or MRI films whatever done to exactly advise u what should b done. but definitely it's not dangerous.