Does it HAVE to be a scroll he can draw on? i mean there are metal, wood, stone or hell flesh if he is a tatto artist too. hah. but scroll often are made of hide isn't it? could be interessing if he got a scroll purely made of dragon scales.
I did a quick scan of the first couple volumes to see if there was any insight as to whether he could do that, and unfortunately I think the answer is that he's just a fucking idiot who hasn't tried Drawing anything that isn't a summon without using a scroll since he was a kid.
Even before the incidents with the Basilisk Dragon where he discovered he could Draw creatures and then with the Cult when Aria likely severed his mana nerves, he had issues using his mana for magic that wasn't Drawing. He apparently figured out using scrolls as a work-around because, being the son of a magic shop owner, he had access to blank scrolls already loaded up with mana, so by Drawing an item (initially potions, later elixirs) on the scroll he could create it primarily using the scroll's mana. But he seemingly lost even that ability when his magic nerves were damaged after the Cult incident, which were only repaired relatively recently by Prol. It apparently just hasn't occurred to him since then to take off the metaphorical training wheels and try something that probably would have killed him as a kid, now that years of doping on elixirs has increased his mana pool to an apparently never-before-seen level.