Use collar will definitely be helpful for him to maintain the good posture. And also cervical pillow can be used which might make you to have better posture of the shoulder neck and arm.
Neck tilt: from the sitting position, tilt your head down so your chin touches your chest. Hold this position for 5 seconds. Return to the starting position and repeat. Do this five times.
Side-to-side neck tilt. From the same starting position, tilt you neck toward one shoulder, leading with your ear. Hold for 5 seconds and then return to the starting position. Do this five times on each side.
Neck turn. Look straight ahead, then turn your head to one side, keeping your chin at the same level. Do this five times on each side.
Neck stretch. Holding the rest of your body straight, push your chin forward, stretching your throat. Hold for 5 seconds. From the same starting position, push your chin backward and hold for 5 seconds. Do the forward and backward stretch five times each.
If any of these exercises cause severe pain
or weakness in your hands or arms, stop right away and talk with your doctor.