It’s possible that your rabbit’s feet aren’t stained at all. Their fur can change colors after a heavy shedding or molt. That may be what has happened in your rabbit’s case.
To see if it is really a stain and not a change from molting, take a white wash cloth and dampen it, then rub it over your rabbit’s feet where there is color change. If nothing comes off, then the color change is from shedding not from staining. If you do see stains, you should try to figure out why he has them. Is there newspaper in his cage? Than can cause staining. Could his feet be stained from urine or poop? Do they feel wet to you or have any odor? What kind of litter are you using and how often do you change it?
If you do determine that the color is from some staining and not shedding, you may not have to do anything to him if it’s light staining, but you should figure out the cause and see if you can eliminate it.
It’s always best to avoid wetting or bathing a rabbit, but here are some instructions about cleaning a rabbit’s messy bottom. You aren’t faced with that, but it may be helpful.
http://www.bio.miami.edu/hare/buttbath.html