Changing position. You need to help him/her changing positions, no matter what. Avoiding sitting in one position more than 2 hours. The seat should be soft.
Sores from sitting. Sores from sitting in wheelchairs are due to all three types of pressure injuries: pressure, shearing and friction. Pressure can be releived by alternating positioning; shearing can be prevented by careful attention to lifting oneself before moving into a new position; and friction can be prevented by understanding and correcting any rubbing issues.