What Causes Sharp Pain And Numbness From Feet To The Hips?
I read your question carefully and I understand your concern.
Since he is a diabetic it is well possible that the symptoms be due to diabetic peripheral neuropathy. However every hypothesis should be confirmed by appropriate tests when possible. Peripheral neuropathy should be confirmed by nerve conduction studies and when there is doubt lumbar spine MRI may also be required to exclude spinal canal stenosis. So without knowing the results of such tests it's hard to be sure about the diagnosis, can say only that it is well possible.
As for treating it Gabapentin is a good first choice. I do not know what dosage is he under, if low and not working it can be increased, can go up to 2400 m/d (though I usually do not go over 1800). If it still doesn't work (or dose increase is not tolerated) than the other option are antidepressants used for neuropathic pain such as Duloxetine. It is also a first choice drug and in addition it can be combined with Gabapentin.
I remain at your disposal for other questions.