Any counter top installer knows full-well that providing and installing a new counter top where the old backsplash is to remain requires the new counter top to be scribed to fit to the old backsplash and walls within reason. In the case of granite this is more difficult to do but none-the-less the way it is to be done and should be part of the installation. This is an aspect that should have been discussed up front because these issues don't always work out well.
If you decide the leave things as they are then you could use foam-rope (called foam caulk) to pack the space. Then use caulk from a tube to fill the space on top of the foam. The foam will keep the caulk from sinking into the abyss and allow you to tool a nice finish. Caulk would be the product to use because the grout won't stay and will crack quickly.
I too would call the genius that did this and let him fix it.
Granite tops are costly but the truth is they are high-profit, so he has money to make it right for you.