Hi :wave:. If he ate just the tinest bit of grass I doubt it would have been enough to cause an upset tummy but then it depends on whether he's used to any green foodstuffs at all. What else does he eat? Is he on a pelleted food? How much of it does he get per day? How much hay does he eat? Fresh veg? One of the main causes of a dirty bum is too much 'hard' feed such as museli mix or pellets.
If his diet is fine then I'd look towards a physical problem. At such a young age, he shouldn't have problems 'making ends meet' :shock: but that's not to say he shouldn't be checked out by a vet.
I'd suggest lukewarm water to soften the hard poo and gradually tease it off as it softens. Don't be in too much of a hurry as bunnies have very delicate skin and it tears easily. Slow and gentle is the way to go. Make sure he's dried off as much as you can, perhaps with some kitchen roll or something similar.