>>my plan tomorrow, during the first force feed, is to use a
>>spoon to pry his mouth gently open.
Probably not a good idea from what I've gathered and I would discourage that method.
Check out these pages to understand what you're dealing with:
"Scaling of Morphology, Bite Force and Feeding Kinematics
in an Iguanian and a Scleroglossan Lizard"
http://jan.ucc.nau.edu/~kiisa/pdfs/Meyers et al. 2002b.pdf
There is evidence that bite force increases with reptile size/maturity and juvenile bite force is limited by skull/jaw strength and not muscular strength. Forcing the mouth open may harm the animal severely/fatally.
Here's an interesting site with skull and jaw details so you can see what you're dealing with. It's a fully digitized and interactive model of an animal skull.
http://www.digimorph.org/specimens/Uromastyx_hardwickii/
From the geometry of the jaw and teeth rows, it should be clear that there's nothing that will open that jaw witout harm besides the animal itself. We pushed our fingernails in at the back of the jaw where the large teeth are exposed behind the beak. Once that happens, it's easy but it can take a couple minutes.
My advice- find a friend or visit the vet. My vet offered hospitalization but I opted to find a way myself. Also, get a good scal so you can accurately track the animals weight. I weigh mine every day so I have a good idea of what goes in and comes out.