Maybe give this thing a shot.
I have a chewer. He's got two of these things, and he can't get through either one, but he's managed to get to the marrow... tasty stuff.
Other people have told you about cow bones. These might be a good option, but the antlers are stronger than normal bones, and would tend not to splinter as rawhide does (and as some people would believe cow bones do).
In that case, I give you this thing. Now, normally, I don't need to go this far in the chewable library, but in this case, it appears to be a must.
What it is is a 100% natural elk antler. This tends to be one of the elk's hardest bones (yes, there be marrow in there) as it's used to fight against other elk, mark trees, etc.
I have gotten one of these for another eater/chewer of things. She can't seem to break it.
Uh, oh, my dog eats deer antlers. They're a super-popular dog chew! Maybe that's why she's so fast...
My pit bull does that too. Antlers are the way to go – but make sure it's an elk antler (like this); he ate about 6" of whatever kind of antler they sell at my local pet store in a single evening.