I don't expect him to make the team outright, but hasn't it basically been confirmed that he's getting the 9 games? I don't think that's a "might" or "maybe" thing.
What physical tools are we looking for? He's the same size as Marcus Johansson, bigger than a few guys on the roster. Look at players like Seth Jarvis who made the jump early without being particularly strong or aggressive (Jarvis is still listed as being smaller than Heidt), Nils Hoglander made the NHL pretty early despite his diminutive size, Logan Stankoven is doing well so far.
Point is, I don't think it's his size or physical tools that hold him back, it'll be the mental processing of the game if he doesn't make it. Can he think fast enough to keep up with the game, know where to be to receive passes, does he have the vision to see the ice well enough to avoid being a victim of size/strength, does he know where to put the puck, can he make the smart, efficient plays when there's nothing better available. It's that kind of stuff that will determine whether he sticks beyond 9 games.