The inside of a dragon is not a known destination
he's claiming that since he can see it, it's known to him
In addition to enkryptor's answer, I'd also argue that the destination is not known to the PC, as per the description of Teleport. It has a table of levels of familiarity on PHB, pg. 281, including Permanent circle, Associated object, Very familiar, Seen casually, Viewed once, Description and False Destination. Let's look at each of them.
Permanent circle: The inside of a dragon is not a permanent circle.
Associated object: The inside of a dragon could be argued to be associated with the outside of a dragon, but the spell description defines this to mean "that you possess an objected taken from the desired destination", which I presume the PC has not.
Very familiar: This, as per the spell description, "is a place you have been very often, a place you have carefully studied, or a place you can see when you cast the spell". None of these are true of the inside of a dragon.
Seen casually: This is "somewhere you have seen more than once", but I doubt that in this case.
Viewed once: This is "a place you have seen once, possibly with magic", so unless your PC has been eaten by the dragon and survived (and even then, creatures swallowed by other creatures are usually considered "blinded") or has scryed inside this particular dragon, this one doesn't count either.
Description: This one looks promising, since someone can easily describe what they think the inside of a dragon might look like, but the spell description says "you know through someone else's description, perhaps from a map", so this one doesn't work either.
False Destination: Even this one doesn't work because, as per the spell description, "Perhaps you tried to scry an enemy's sanctum but instead viewed an illusion, or you are attempting to teleport to a familiar location that no longer exists".
So my conclusion is that, in addition to what enkryptor has said, they do not know the inside of a dragon, therefore the target is invalid and the spell will not teleport the rock inside the dragon.