I agree with Amy, your horse is not old, if you keep him fit he'll do the camp with no problem!
My advice is work him a little bit more every day, give him a break (walking) regularly. Maybe you can start working him twice a day (if you have time) with a hack and a workout in the arena!
During the camp you can support the tendons like Amy said, and give him something to help him recovering faster (some suplements, not sure wich ones are the suitable one)
Hope ot helps
First of all you need to know that your horse is not old at 15 years old if he is well maintained.
You need to make a long warming up (with a minimum of 10minutes on the walk), do a divided work (intensive work than 3-4 minutes on the walk with distended reins).
You can do a lot of walks or trotting, it will be good for his physical condition and also his moral.
The most important thing during this camp is to watch at the recovery, you can put clay on his tendon or even standing wrap.
Finally, don't forget to make an active recovery when you end your class (5minutes of trot with distended reins and then a long walk with distended reins and you can release the girth).
I hope i could help you :)