Although it would be difficult (and expensive) to do in the fall/winter, I highly recommend water therapies for older dogs. Especially if you're noticing any kind of limping/lameness, this will be particularly helpful.
You can take him for a normal swim in any pool, or rent time at a facility that has underwater treadmills. The water helps take a lot of the weight off the joints, making it easier for the dogs too keep moving, but the physical action helps to keep muscles and joints limber, as well as helps promote cardiovascular health.
The suggestion to walk him more frequently for shorter durations, is also a fantastic idea.
Do keep in mind that any muscle that gets used, will also need to be relaxed. Spend some quality time with your old man, massaging and stretching him out. Unless he has any particular issues, the massage should not be anything complicated or deep - a simple firm rub in small circular motions will do just fine.
Older dogs will also typically have decreased circulation. After the massage, in a sweeping motion with a very light touch, "flush" the fluids towards the heart.
For example, "sweep" the leg from the tips of the toes up to the shoulder or pelvis; and from the tip of the nose, across the face, then down the neck.
Try stretching each leg forward, backward, and across the body firmly for 30 seconds at a time, making sure to release very slowly in order to avoid a reflexive pull back and tensing the muscles again. Try this with the legs folded, as well as straight.
With your dog in a standing position, have him follow a treat with his head. Bring it down to the floor, then between his front legs and back towards his hind legs. If he keeps turning around before getting a good stretch, start closer, and let him gnaw on the treat for a few seconds. You may also want to enlist the help of a friend in holding his back end still as he only moves his head and neck.
In a similar fashion, you can have him bend side to side by holding the treat near his face, then bringing it around slowly towards his hip. I do this one by sitting on the very end of a chair, and holding the dog's hips between my knees, as I guide the head down the length of the body.
Just keep him moving.

If he's still instigating play sessions with the other pup, then you're doing a great job.