Maybe he could say that due to personal developments he felt he had to move on at the time, and now that these needs have been forfilled he would like to know if there is an opportunity to regain emplyment within the company once again.

He doesnt have to say what the personal developments were....he could say a white lie and say that at the time his 'head' wasnt in the right place IFYKWIM and he thought it would be better to remove himself rather then display the negetivity and stress he had in his work.