he will need it cleaning off, if you don't he can get fly strike, which means flys can lay maggots in the poo, you really don't want that to happen, the maggots can eat into the flesh.
if he is a wriggly fellow, get someone to help you, wrap him tightly in a towl and tip him on his back, hold firmly and get the person helping to wet cotton wool or use old flannels using warm water, try and soften it up and clean it away, it should work if you keep wetting it down, you could cut the fur around, but i am obviously worried you may cut him if he wriggles so not a good option, failing this take him to the vets for them to do it.
you should check him daily for this problem, it shouldn't build up if you keep an eye on it, be extra careful now the weather is warming up, you will really have to watch out for those flys, keep the hutch as clean as possible, cleaning out poops daily.
did you know you can litter train rabbits, then you only have to clean the tray everyday.
hope this helps.