on the assumption that you have clearance from his physician, I would suggest that you contact his physical therapist and ask for advice there. Web sites can give you a lot of general information but the PT should be more familiar with his particular situation and ought to know the history of the back problem.
When I have a new client in a similar situation, I always do my own assessment and share this with the PT. I then start with the PT exercises that are familiar to the client and add only those of general conditioning that get the PT stamp of approval. As you continue to work with the client you’ll get a better feeling for his abilities.
Getting started with a client with such issues is always tricky because you do not know him yet. Always err on the side of safety.