[@thewizardguy]
I have it being iron due to his hybrid nature
Also some vampire lore have weaknesses to iron so it's also a nod to those

I figured since he didn't have a weakness to sun and he didn't have a weakness to silver his weakness would be something older and more inconvientent 
Like for example cars....

also this paragraph:
"It wasn't until humans began realizing certain things that they were weak against that they had any hope of survival, chances of fighting back. Silver, holy water, crosses bared by those with unwavering faith in the Lord, fire, blessed objects and iron, and other such items harmed or even killed them. Finally they had a fighting chance, and thus the initial order of vampire slayers known as The Covenant was formed. At first it was a small order of priests and able bodied men dedicated to the sole task of eradicating the Nosferatu, or at the very least hold their ground."

unless I was wrong to base his weakness upon this 
In which case I can come up with a different one