hi,
first of all you should understand that with proper positioning you can prevent only bed sores not contractures Give him/her Passive ROM exercises minimum twice a day to prevent contractures.
must change patient's position after every two hours to prevent bed sores. when you put him/her laterally place a pillow between his knees(both legs).in the supine position place pillows under his arms to prevent edema and swelling.
best regards
umer physio