I have great success with reducing cramping in clients with two things.
One, stretching the affected muscle frequently. This has been especially helpful with calf cramps.
Two, drinking a sport recovery drink.
For clients that experience cramping at night, they tell me keeping a sports drink on their night stand for this is helpful. And gentle stretching of the muscles affected right before bed or upon being woken by a cramp also helps. The gentle stretching also can be helpful in endurance events. The time lost to stretching beats crumpling in a heap from pain.
There are some other techniques that you can learn, but I would not be comfortable trying to explain them without including a hands on component with actual cramping athletes/clients.
Electrolyte solutions may not be enough sodium and potassium for him depending on the climate he trains in and the amount that he sweats. Personally, during my marathon training, because I train in sunny California, I need to not only supplement with electrolytes during runs but also over salt my food and add salt to my water throughout the day to prevent cramps.
I agree with Marshall and a movement/gait evaluation might solve the problem. In addition to what was mentioned before, compression socks, extra salt, foam rolling, massages, better shoes, and maybe incorporating bare foot running might add to dexterity of his feet and aid in preventing cramps.
Over training might be something to look into as well. Too much milage and speed could be the cause as well.
There are some good suggestions here. But, the first thing I would do to rule out a likely possibility that his calves might be compensating for poor movement elsewhere in the body, is a postural and structural exam.
Recently, one of the sports med. editors of a national running magazine published that she had similar cramping problems with her hamstrings for many years. She had seen many doctors, PTs, coaches, etc… and no one could explain why. Then, during a proper structural movement evaluation, it was discovered that her glutes weren’t firing properly, causing her low back and hamstrings to compensate for hip extension by doing all the work. Her hamstrings would fatigue and become over worked and then cramp.
One example, is that your client may not be getting full hip flexion and extension while running, causing them to compensate with a more powerful plantar flexion to create forward movement (ie bouncing off the calves).
The first thing we do with every client at our facility here in Tumwater Washington, is postural and structural evaluation to determine what weaknesses or possible injuries may occur during future training or even daily activity and begin programs by addressing these.
If your client is properly hydrating and nourishing themselves, this is the first step I would take.
I agree with the previous answers regarding nutrition, hydration and foam rolling and would like to add that I had a client who tried everything and still got leg cramps on those long miles. He started wearing full length compression running tights and no more cramps! Compression socks would probably work just as well for your client if he is only getting the cramps in his lower legs. It is worth a try! He might want to try taking more salt during the long runs too. I find that for half marathon distance I am fine with regular electrolyte replacement drinks but once I get up into those longer marathon distances during training (and races) I need more salt than is in any of those drinks. I take those little sachets of salt with me and pour half into my sports drink at about mile 12 and then the other half at about 18-20. Some people I know just eat the salt right out of the packet and that works too for those who can stomach it!