This is a fairly common illness in older dogs. They can also get the opposite illness Addison's disease. Cortisol level goes too high its Cushing and too low its Addisons. Treating Cushing's Disease in Dogs (http://www.fda.gov/forconsumers/consumerupdates/ucm151209.htm)

Read the site ballenger posted above, it is very informative on what the disease is and how it is treated.

As for the elbows where does he sleep?
Not just at night but where does he lay during the day?

Larger breeds even without cushings can get terrible callouses on their elbows and even rump from laying on hard surfaces regularly.
If he likes to sleep on somewhere that has tiles or floorboards invest in a nice thick mat for him to lay on, or even an old quilt would give enough padding to help stop the callouses forming.

One of the effects of Cushing is can adversely affect skin and connective tissue. Is you dog being medicated by the evt, both Addisons and Cushings can be treated