GrreatTeacher:
Planking works the entire core muscle group as well as most of your other muscle groups as stabilizers. It definitely helps build a strong core. However, because planking requires your legs and torso be stretched out and suspended between your two extended anchor points, there is no support for your back and if you have weak abdominal strength to begin with, you will not even have their support in keeping a straight spine, so I do not recommend planking for spinal injuries.
Rather lie on your back so your spine is firmly supported and do bent knee leg raises for your lower abdominal wall or hanging leg raises but be sure to tilt your pelvis upwards only slightly. For the upper abdomen do "cheat" crunches. This involves lying on your back, folding your arms across your chest, bending your knees with your feet planted firmly on the ground. Now lift your scapulae off the ground no more than two or three inches in a slight curling motion, exhale and concentrate on squeezing your abdominal muscles and hold the position for three seconds, relax letting your scapulae drop back down to the ground as you inhaleso your entire upper body is in full contact with the ground. Repeat.
You can google or YouTube these exercises to get a complete picture on the correct form for performing these exercises.
Calcium carbonate is king as far as bone supplements go. Glucosamine will help slow down cartilage and disk wear and tear. It is possible to increase bone density right up until your 70's with moderate resistance exercises.
Is it his actual vertebrae that are deteriorating?
If you have any questions, feel free to ask.