User blog comment:B214/High School DxD EX/@comment-24053170-20170109193427/@comment-24053170-20170109221159

He did the first time, but it says that he went on to have children, which implies he had more then one with Mine. The first one was conceived before the transformation, but he had more with Mine after they settled down and lived together.

Though if you have read the series you will know its actually possible, as one of the most important side characters in the series is Half-Danger Beast.

Still, we will have to wait until the author gives a confirmation to the fact, though it is directly implied he is stuck that way when it says he returns to his village which caused a commotion because of his draconic form.