Board Thread:Questions and Answers/@comment-26572409-20150708123245/@comment-25121259-20150708211713

The Fullbring from the skin on his right arm might already be in it's final form, as he referred to it as the true form of his right arm (he might have realized it subconsciously). Especially seeing as its form didn't change during the timeskip (during which he and Inoue trained to get stronger). Also, there actually was an explosion (a lot of smoke came out, though that might have been from the enemy's attack ?) when his right arm turned into the shield form and seeing as he has a very durable "body" it's possible he simply didn't need the aid of another Fullbringer (the completion of the Fullbring only might ruin the body, there is no guarantee that it will happen for sure).

Of course, there is still the rest of the skin and perhaps his grandfather's coin ?