I wouldn't get worked up about it unless you hear cracking constantly as he's walking, just moving in general, or while being ridden.
The cracking or popping sound is thought to be caused by either escaping gases, the tendons moving out of place thus making the ligaments tighten (both can make a cracking sound), or rough surfaces caused by arthritic joints.
We get the same thing when we crack our knuckles or stand after sitting in the same position for a long time.
If you feel that your horse truly needs help from a supplement, I've found that pure MSM helps well. It depends on the horse. It helped my pony to the point where he no longer limps from the arthritis in his knees. While you can still occasionally hear a crack or pop with my mare, but definitely not as bad as it was before. It could take about a month for you to notice any difference, if you decide to get some for your horse.
Whitetrash is right on. We have a horse that is 23, and his joints crack. MSM has helped alot, and we always make sure that we stretch his legs real good before working him or riding him. Pick his front legs up, pull them slowly up towards his chest a couple times, and them straight out. His hind legs should just be pulled straight out. NIce and slow so you can feel them stretch. The more you do it, the more used to it he'll get.