He was on a very high dose of oxycodone; his brain will eventually recover, but it will take months and months for that to occur-- I would say 6 months at a minimum. His 'taper' was much too fast; typically a controlled taper would take a year or more from that level of narcotic.
Everyone who takes pain pills doesn't get addicted to them, but there is always that possibility. If he did become addicted-- if he developed the state of mind where he thought about opiates constantly and had trouble controlling his dose, for example-- then there would be permanent consequences from the opiates. People with 'opiate dependence' have a chronic, permanent illness that consists of many different aspects. For example, the person becomes very aware of their physical symptoms; the person becomes very attached to the medications; the person sees the world with him at the center, and becomes spiritually empty. For this reason, treatment of addiction includes a spiritual component in order to replace what has been lost, and to help the person see a 'higher power' or a purpose to life.
Finally, he may have a depression along with the other conditions. A depression often leaves a person feeling 'hollow' and empty.
At a minimum I would consider getting him to see a psychiatrist to address his depressive symptoms. Depression is treatable, and he should at least consider such a TEMPORARY diagnosis before making life changes.