It might be a callous. But it's kind of hard to tell from that picture. Is the area inflamed (infected)? Does it hurt him when you touch it? If the answers to those questions is yes, then this is the beginning of sore hocks.
For treatment this is what I found, this tells you about sore hocks and callous treatment. It's a very helpful article; Raising Rabbit - Rabbit Feet Problems
What breed is your rabbit? Plush Lops and Rexes tend to get sore hocks more. If he/she isn't a plush furred breed, then you should probably make sure that your bunnies space is always clean, unsanitary places lead to sore hocks more than anything. If your rabbit is on wire (which is fine, actually a better option to avoid sore hocks because they're not sitting in their filth) make sure they have a resting mat (you can find them on amazon for next to nothing). If your rabbit is in a plastic bottom, hutch, or free roam/a pen. Make sure you clean a lot and have clean fleece for him/her to sit on.
Hope this helps!