Work with the doctors concerning what exercises would be best to avoid. If he has been going to physical therapy, contact that person to see what they have been doing and use that as a place to begin. I would think that some large muscle groups and walking would be a good place to begin. He may need to get the muscles to "re-learn" stretching and strength, so begin with only a few of each exercise, maybe 5 reps at most then build up as he gets better. Make sure he works the lactic acid out of the muscles after each session to avoid cramping. Steam rooms and whirlpools are wonderful for this. Ask him to get a list of "qualifying" physical tasks he needs to achieve in order to return to work and plan his exercise progression to achieve his goal. He may very well be able to return to work if his thinking functions are intact and he can get his body into physical shape. He may not be able to do the "street" jobs, but there are other less strenous jobs he can probably perform. This is between him, his doctors, and his employer. Don't discourage him from his goal, it may be what he needs to keep going every day. If his goals begin small, he will feel better with every small achievement that takes him closer to returning to work. When he reaches the goal of say, 5 reps without residual pain, give him alot of praise, and tell him his first goal has been met and it's time to "move on". His self-esteeme will increase with each goal met, however small. The goal of "getting back to normal" gives him power over his life again. good luck
You might want to have him ask his neurosurgeon what exercises are appropriate.
Surely he must know that he won't be able to go back on the streets as an officer?