Does he take a pottasium suppliment? It could be cramping, although that typically happens in legs form what others have said.

It could be form the weight. If he is still walking like he has that extra weight, his posture may be pulling on his back. If you can encourage him to start a program to strengthen the back muscles as well as abs. Abs support the back and protect it form injury. A pilates program could be great, I don't know if it would be to femanine for him though (men can be funny that way). If he is already exercising, make sure he pulls in his abs tightly during the moves to support the back. Slow controlled movements with weights while the abs are in tight also help focus the exercise on the muscle group wanted and not rely on momentum.

Does he drink enough water? Could it be kidney pain or other pain related to dehydration?

Did he have the pain since starting, or just the 45 pound mark? It could be some event at the 45 pound mark that injured his back, he just didn't relize it at the time. He might want to see a doctor to be sure there isn't a slipped disc or cracked vertabrea.