Thank you for your question.
Subarchnoid impingement in the report means the disc is impinging on the membrane that is surrounding the nerve
. It does not necessarily mean the nerve is compressed. It is not clear from your description why this MRI
was obtained. If L4-5 was fused in 2000 and fusion had been successful these all changes should not be seen. Whatever the MRI report if he is not having any symptoms you do not need to worry. The treatment decision will depend on his symptoms.
Wish him the best.