He has hydros...no boot required on it. Regular cable brakes have that boot with a clip holding the rotor on. Your axle isn't all the way flush with the rotor hub. Looks like you got some rust in the splines. Id take the whole axle off, wire brush the rust off the splines, put some anti seize on the splines and reassemble. Just takes all of ten minutes to get the axle off. You want to pull everything out of the right side when you pull the axle. All you have to do is:
1. Put rear end in the air
2. Loosen axle nut in center of wheel
3. Loosen wheel and take it off
4. Take axle nut off
5. Loosen tension off the chain and take it off the sprocket
6. Loosen the 2 axle nuts and take them off the axle
7. Pull Sprocket hub and sprocket off the axle
8. Loosen caliper and take it off the rotor and lay it to the side
9. Pull axle thru the carrier
Once off, you can get the hub off with the rotor on it and get the rust and corrosion off and put the anti seize on it and then try sliding it right on. It should slide flush with the stop on the axle without any force. Then reassemble...you may want to also do this to the splines on the sprocket hub as well.
It's a rather easy fix...