By your husband’s complaints, it appears that your husband has compression of the S1 nerve root at the L5-S1 disc. It is a possibility that the compression could be the L5 nerve root if the spur or herniation is in the foramen but that should not matter to you.
He already has had 3 epidural injections with only temporary relief and has had pain for over three months. He is in extremis (severe pain that prevents him from walking).
Based upon what you have told me, he should be a candidate for a surgical decompression of this level. Find a competent spine surgeon and get a consultation. Surgery should have at least a 90% chance of relieving a good portion of his pain.
Dr. Corenman