User blog comment:B214/Question and Answers/@comment-9063800-20130806230633/@comment-180.194.30.37-20130807040546

From what I think..

He cannot do a full body transformation because his current body is just made out of the flesh of Great Red and that doesn't gave him powers of transforming, him just becoming a "dragonoid"..

And also I think the part where Issei's arm transformed to a dragon's does not apply anymore; Why? Because the arm that he sacrificed was from his former body, not the body he has now.