How can I wash my Dad's hand?

My dad has dementia and his hand is closed shut from lack of use. His aid was suppose to exercise him but he gave a hard time and now we have a big problem the hand is shut tight we put a stress ball in his hand to keep it open and from the ball being in his hand he got a pressure sore on his finger.How can we clean this his hand, smells sometimes and he's in pain with it .Please any advice? Very difficult when the hand wont open.