Hi, Welcome to HealthcareMagic Forum.
Your Dad's report suggests he has Lumbar Spondylolisthesis
which is a condition in which a vertebra in the lower part of the spine slips out of the proper position onto the vertebra below it causing nerve compression and Pressure symptoms like Lower back pain, Muscle tightness, thighs and buttocks pain, muscle Stiffness and Tenderness in the area of the slipped disc, in your dad's case it is L5 and S1 vertebrae. In addition to this, the foramina or as we call it spinal canal
is narrowed which is called spinal stenosis
. Consult your Orthopedist, he will explain it to you in a better way by means of diagrams.
The treatment includes both surgical and non surgical. The surgical treatment
includes, fusion of the slipped disc if the symptoms are severe, A brace or body cast should be used after surgery. He will need follow up x rays to see if the vertebrae are changing position over time. Non-Surgical treatment includes, Analgesics, Physiotherapy
and a back brace. Your treating would know better.
Hope I have answered your query, I will be available to answer your follow up queries.
I Wish him Good Health, Take Care.