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.