Type 2 diabetes

Could be neuropathy.

What were his last HbA1c levels and what kind of diet are you following for control?
It might be that he'd benefit from tightening up a bit.

I had a touch of this when I was younger
https://www.mayoclinic.org/diseases-conditions/cervical-spondylosis/symptoms-causes/syc-20370787
A bit of chiropractor manipulation cured it instantly. Is he sleeping with several pillows under his head? Or falling asleep sitting up?

Other causes could be poor circulation, which tends to get worse on cold days. Another possibility is a vitamin B deficiency affecting the nerves, but this would probably not be just the hands.

But GP is the best route to take since it could be due to infections or viral.

As said in the previous post, diabetes neuropathy is a candidate that needs to be considered since it goes with the territory.