Do you have another rooster or any sharp fencing where he might have ripped it? I would leave it alone so it stops bleeding, and let the blood act as glue for his comb. If he shakes it a lot and keeps it bleeding you can take a sharp pair of scissors and trim off the loose part, using an ice cube to make his comb cold to help control blood less. But I would try the conservative way first. The separated part may eventually shrivel up and die off, but that is fine. It is painful, and he should be separated while having red blood there and being subject to pecking. Let us know what happens.