Hello~It sounds like a circulation issue to me. Is he on any medication for the diabetes? Maybe sleeping with his legs on a pillow so they are elevated a little bit would help.
I would contact his physician and tell him what is happening, hopefully, he/she will have some idea as to how to help this issue.
This is possibly diabetic neuropathy. This is caused by chronic high blood sugars causing damage to the his nerves.
There are medications that may help.
However, the best way to reverse this is by having very tight blood sugar control (ie. keeping blood sugars in the 70 - 120/140 range all the time). To do this it is necessary to control his diet (low carb is best in terms of reducing blood sugars) and take medications appropriately. Please note that if he is able to keep very good control the pain may temporarily worsen (regrowing nerves can be painful) before it gets much better.
Of course, this advice does not replace seeing his doctor and a proper diagnosis.
Hope this response will be helpful.