Lunging him is a good idea, but take it easy. I wouldn`t do more than 20 min to start with. The first couple of times just walk and frequent changes of rein. Build it up slow he is an older horse so it will take time.

 Well as he has been riden before, he should know all the aids. However, a little re-schooling will be needed. The first time you ride him, ride him in an enclosed area with another horse. Just do some walking, maybe with a friend on the ground to help you out. Check he knows the three 's's, as these are the most important (Steering, Slowing down and speeding up). He may need reminding so you may need to use your voice to help him out (doing some lunging/ground work building up to this will help him learn the voice aids). Before each session, set yourself small, simple goals at first, eg. acheiving a nice sized circle, being able to do transitions easilly ect. Be aware that he will be very unfit and may only be able to do little bits of trotting. Also try hacking out as hills/ different terrain will help him with building up the muscles he has lost.. At first be sure to always have a friend around as he is getting back into it so may get a bit excited so help may be needed. Once he has the hang of things and is a little fitter, do lots of trotting up hill (gradually increasing this) to build up his muscles. When he is well capable of trotting with controll maybe start adding some canterwork, but be aware that his artheritus may limit this. Hope this helps!