Spectacles for children with CP in wheelchairs

Hi 
i have a 14 year old son with CP and is s severely disabled physically. Finding the right frame for his glasses is a challenge.  This is because he has a high muscle tone and is headrest gets in the way of the glasses..We have tried different types of frames but nothing has worked. He really needs to wear his glasses as if he doesn't everything is blurred. Any suggestions please..