Regarding how you should ride him, I would be walking him for at least 2 months on a fairly long rein. Active walking, starting with no more than 10 minutes for the first couple of weeks and building up to 30-60 minutes. Start out working him on a nice flat surface, and eventually build up to rides out over varying terrain. Being an older horse, you're going to need to be extremely aware of not pushing him, or you will damage soft tissue and joints very easily. More so than with a younger horse.
After a couple of months of walking, start to introduce very short periods of trot, on straight lines and large curves. Only for a minute or two at a time, with large recovery periods of walk between. As with the walk, you can slowly build up the time spent trotting before introducing canter in the same manner.
I can't stress enough how important it is to take it VERY slowly - it is like asking an unfit, slightly arthritic 60 year old man to start training for a marathon. If he went out and ran hard on the first day, he's got a very high risk of doing an injury.
It is your job to slowly and gently condition the horse's body (particularly the limbs) into work, and being an old horse, this will take a long time.