well, i would start slowly anyways. the horse needs to be reconditioned. If he ignores the small jumps, just do some ground pole work. You will see how fast he remembers things and how in shape he is. I think it shouldn't take too long to get him back jumping if he's been doing it all his life, but taking your time with it will be the best thing. If he gets bored with what you set up for him don't hurry on to the next higher level, just take it slow =)
Also, maybe keep it to once a week or so and build up. If he's 19, jumping is going to slowly be bad for him because of old age, especially if he's been jumping most of his life. I know that there are some horses who jump way past 20, but if you want the horse to do so too then you need to keep things slow. I've ridden older jumpers, and you'll be surprised by how fast they go once they go.
Other than that, i wish you the best of luck. Congrats to the horse for getting over his illnesses, and i think as long as you keep things in moderation you should be fine. You can also consult a local trainer or coach to help you maybe, nothing beats hands-on advice!
hope this helps =)