Hello there, it was very nice of you to take this bunny in
In terms of grooming, I would attempt to give him a gentle brush, don’t feel the need to brush all of him at once if he doesn’t want to tolerate it or it is stressing him out too much. A little bit at a time is fine and focus on the worst areas first. If his butt area is clean aside from being wet he shouldn’t need a butt bath, but if he has sticky poop stuck to his bottom, you will need to gently bathe him in some lukewarm water, only deep enough to cover his feet and then just bathe his butt to clean off the poop. Don’t use any human soaps as they are too harsh on rabbit skin.
Good idea on worming him since he is so thin. Hay and definitely pellets are probably the most important for now, the hay will get his gut working properly again and harden up his poops, which should also help with his wet butt. Depending on how your employee was keeping him may explain the wet bottom, but if it persists after his poops have hardened up and he’s been on the new diet for a while, he may have a urinary tract infection and will need to see a vet. For now, keep doing what you’re doing, encourage lots of hay eating and the pellets will be good to help put on weight, though be careful not to feed too many at first since his gut won’t be used to them and it may cause more upset.
Does the cage he is in have a solid section of flooring for him to stand on? As a general rule wire floors are bad for feet, but in particular for a rabbit which is very skinny and has no padding between bone and skin, wire floors can be quite painful and you may find the fur wearing away and causing sores on the feet. I’d recommend housing him in something else or making sure that at least half of the cage has a solid or soft floor that he can rest his feet on. Obviously your cage may have this already, but I can only comment based on what I can see in the images
