User blog comment:B214/Question and Answers/@comment-9063800-20130806230633/@comment-5185939-20130807184812

In Vol 14 it's revealed Issei can turn any one part of his body into a dragon form. This increases the strength and durability of it enormously. At least on par with when he promotes to Rook (and that's without the promotion.)

I'd say it's entirely possible that, with more power and experience, Issei will be able to assume a fully dragon form, though it would probably be a humanoid dragon like Grendal.

However, he still can't return the body part to human form without the excess magic being drained by one of his girls, so he'll probably want to master shifting back and forth at will before he tries to do the full body transformation.