Sounds like a typical case of sciatica. The referred pain is due to compression of nerves in the lumbar area. Sometime in the past he lifted something too heavy, or lifted improperly. He needs an MRI (Not a CT - an x-ray is a waste of time), and to avoid lifting and when he does lift, to lift properly. Once properly diagnosed, a physical therapist, directed by a physician can teach him some specific exercises that will relieve the problem, and that should be square one. Count on half a dozen sessions. The exercises are not intuitive. Pain killers don't help much. A TENS device is sometimes prescribed for chronic pain. Surgical resolution is a last choice. The procedure is called a laminectomy. In Europe they have had some success with injection of ozone into the space between the vertebrae.
Welcome to the forum!
The symptoms that your father experiences, of pain that starts in his very low back, hip area and goes down his left leg to the knee, is very suggestive of sciatica. . It generally happens when you bend or lift things the wrong way. However what can be serious is a slip disc or a bulging disc. The pain can also be due to irritation of the sciatic nerve from infections, injury, pressure from an adjacent bone, tumors etc. Please do not ignore this. The same has been stressed upon by Caregiver. Numbness comes at a very late stage where the nerve is compressed to such an extent that all sensations diminish and there is numbness.
A neurologist is the specialist you should approach. Maybe your PCP can help get an early appointment. A MRI, as suggested by Caregiver will be required to access the situation. Some patients benefit by conservative treatment, others need varying degree of traction and few need surgery. Physiotherapy is definitely required.
As advised by Caregiver, your father should refrain from lifting weights, bending and straightening with a jerk, jerk rides on uneven roads etc.
Hope this helps. Please let me know if there is any thing else and do keep me posted. Take care!
One of the reasons to recieve prompt medical attention is that prolonged compression of the nerves can cause permanent damage, and result in a condition known as "drop foot". You want to head this complication off at the pass. As Drnee mentioned (which I should have) is that riding in a vehicle on bump roads really agravates the problem. If your father must travel, get an inflatable cushion to sit on, or a foam wheelchair pad. Dehydration is another co-factor. Ocasionally, physicians prescribe steroids, however these substances cause a host of other problems. In the short-term, for relief of extreme pain, they may be an option. As drnee stated "do not ignore this". It is only going to get worse over time. Often the end treatment is one or more laminectomies, but this should be a very last resort. If you do get this procedure done, have it performed in a facility that does a lot of them.