Two problems with getting an astro camera...
1) He's not using a telescope. He's are just using a 300mm lens.
2) His camera is not a problem. He's just learning and improving. A different camera isn't going to make that any easier.
The D7100 is an excellent camera, and he will see excellent results with it.
If he was shooting planetary, I would agree. Just because of the smaller sensor size (= larger image), and ability to shoot video with more control.
1) Astro cameras can be used with Canon/Nikon lenses, i already bought an adapter that i can place Canon lenses to it, i even ordered another adapter that is better designed to use filter wheel too with Canon/Nikon lenses, so a Telescope isn't a must for an astro camera.
2) Camera can be the problem, it can be the key, or it is nothing, this is all depends on what he wants or what he can do, i placed my DSLR and my Sony A7R to my telescope and took few exposures, then my astro camera, the exposures from an astro mono camera is like right away modified or easier to process than from my DSLR, because my astro camera is more sensitive and designed for astrophotography, so may not find that a big deal, to me it changed my game and i never touch my DSLR or Sony when i shoot astrophotography, so the camera is for everything or anything related to the sky, even the sun or planet, and there are so many astro camera dirty cheap that can do the job.
Indeed, D7100 is a capable camera and can be used for nice results, but i was serious about astro even i am just a beginner, so i didn't waste my time learning with my DSLR, i just bought an astro camera and i never regret it.