could it be some sort of vascular swelling?

does it change at all if he props hands up & doesn't use them for awhile?
color changes?
do the arms seem larger/swollen too or only the hands?

possibly a repetitive use issue??
arms used overhead often over the years??
framing/hammering , drywall/ceilings or above head work , & heavy lifting??

If it isn't Carpal tunnel or something like arthritis ....
They may want to look thru the Thoracic Outlet Syndrome forum stickys if any of the above apply.