What types of exercise should I do to get my horse in shape after being off for two months with a shoulder injury?

A lot of trotting. But not just trotting around. Make sure that you have him on the bit. If you have a dressage whip tap his hind leg about every 3-5 steps but do not let him speed up when you tap him, make sure he stays the same pace. Then take him over cavaleties that are just on the ground to start off with. And start with 6 poles and after every week add 1-2 poles and when your ready make them actual cavaleties
I recommend stretches before each session. Pick the front hooves up as if you are going to clean them, then pull it forward until the front leg is mostly reached out. You may as well do the hind legs too while you're at it. Get the blood flowing :)

Lunging, specifically on the side of the recovering shoulder. Lots of trotting for the first few days. Urge him up into a lope after that. Once it is slow and controlled, he is ready to ride. Once back in the saddle, again, lots of trotting before graduating back into the canter. This should be a slow progress. Best of luck!
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