Hello,
I was diagnosed with congenital spinal stenosis at 23 years old after significant symptoms similar to your brother. Spinal Stenosis is really common with elderly and older people as their disc begin to deteriorate and other calcification etc., occurs in the spinal canal. Plain english-this is a narrowing of the spinal canal which causes increased pressure/compression on the spinal cord among other nerves. This could be a cause, or secondary cause due to some other issue. I would be inclined to ask about this because Spinal Stenosis presents with common symptoms of pain after walking and standing upright as well as sitting, these all add compression to the spine and discs.
An MRI would most likely be the preferred diagnostic method.
I would notify the family MD for his or her consult, they may order MRI and dependent upon findings refer to neuro/orthsurgeon for treatment. For us younger guys, they typically try to avoid surgery so PT and other pain management and rehab would most likely be the first course of action depending on the results.
I am not a MD but I have my own experiences that I am speaking from.
I would definetely reach out to my family doc and ask about spinal stenosis simply because of the impacts to leg pain and