Is it possible to get a accurate assessment of a roof without ripping it off?
He eventually gave us a price of the job guaranteeing that this would absolutely not change.


Ok, so I assume this is in writing, correct?  If so, it's his problem, not yours.  Of course there may be the "fine print" that covers such things and makes the roofer's "guarantee" worthless.

It's hard to say beyond that without seeing the actual contract you signed.

But yes, a roofer should have been able to determine the condition and thickness of the decking before giving you the quote.