To get horses back in shape after long periods of time out of work, generally the two worst things you can do are lunging and going faster than a walk. The combination is deadly on joints.
I would tack him up and hand walk or ground drive him in the walk only and then when you are fairly confident he's still broke, hop on and walk. I would only start short bursts of trot (1-2 minutes tops) after a few weeks, and start out on ground driving, then again undersaddle. After the first 4-5 weeks of only walk and trot, then try short bursts of canter.
If the horse is truly a schoolmaster from Europe he's going to know some stuff so don't cowboy up on him if he offers something like a haunches in or some short steps instead of what you thought you were asking for. He may have a lot of buttons and you have to figure them out. After about 6-8 weeks it should be ok to start really playing with the buttons and trying to figure out what everything does. At this point it might be useful to have the owner, or a Dressage coach come give you a lesson once a week as he should be fit enough to start some basic lateral work and things.
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