Rabbit is not cleaning himself

He is the only one out of my 4 rabbits who doesn't let me touch him. I think he was traumatized when I took him to neuter, it was a low cost neuter and I am still not sure if they used enough pain control. They mentioned gas (forgot the name) but didn't mention any metacam (I don't remember if he was given anything after the surgery)
Anyway, he is 2 yo neutered bunny. Pretty lazy and big, I have to admit. I feed them Oxbow pellets for adult rabbits + Veggies + some treats (fruits, Oxbow treats). I know about not giving too much treats, and I don't, but if somebody doesn't want their portion, he would finish everything. He loves to eat - this is his only favorite activity. They graze grass in the yard.
Its a mystery for me, but my females who are solid brown color don't molt (at least, I don't see it). Both my males, broken white with brown, molt pretty bad. Does it have any scientific explanation? They are all related (parents, who are siblings, and their children)
Anyway, today I decided to catch Paul to pull out some extra fur. I do this regularly to his farther, but Paul never lets me (they are pretty much not confined in the cage). I caught him, and then I saw his private parts surrounding fur is covered with poop and urine. Never seen it that bad in other rabbits. I washed what I could with warm water but it was difficult and I didn't get him completely cleaned.
Is the reason his obesity? I don't know how to feed all my rabbits a good stuff but not him? He already doesn't like me much.

How can I determine if my rabbits have mites?

I have read that obese rabbits are often unable to clean themselves because they're too round to be able to bend over. Maybe you should take away his treats and cut his pellet and veggie portions down, for instance by locking him in a bathroom to eat his rationed share while you feed the other bunnies, and only let him out when the others have finished and you've cleared away the leftovers. He won't like that, but a healthier bunny is a happier bunny, so keep working towards that goal! Be sure to give him lots of fresh hay to eat so he doesn't get hungry.

Signs of mites include dandruff in the fur, or little black specks of mite poops.

Thank you. Do I need keep checking his butt and wash him if he is dirty?

Is he having mushy cecotropes stuck to his bottom? Is he also getting urine soaked fur? Is he just a big rabbit or is he overweight? Does he appear stiff at all and have any difficulty hopping? Does he eat plenty of grass/hay?

Yes, it is important to keep them clean and dry in warm weather, as a soiled bottom leaves them at risk to fly strike, which can prove fatal if not caught immediately. If you are having trouble keeping his bottom dry and clean, it may be advisable to take him to your vet to have his butt shaved. If you are cleaning his bottom, you also need to make sure it is dry, as damp fur will also attract flies.
http://www.rabbit.org/journal/2-12/fly-strike.html

Signs of mites will be excessive scratching, hair loss, and flaky skin.
http://www.medirabbit.com/EN/Skin_diseases/Parasitic/furmite/fur_mite.htm