THe only problem with fresh grass and weeds is if they don't get them regularily, like, if you let him out in the garden just once a week - his digestion can't adjust then. There are no grasses that are harmful, and very few toxic plants that actaully can do any harm. They know prety well what to avoid when they have the chance to do so regularily.
Everything that is new gets ignored. Then they take a test nibble, if that dosn't have any ill effect, they try a little more. That's the way rabbits react to new stuff. I needed months to get some of my rabbits eat pumpkin.
As been said, rabbits need a lot of fiber, human food does not have that. Rabbits are better off with greens we can't digest. Vegetables and fruit can be a small part of their diet, but as a staple food they are too rich. It's like if we would only eat meat, and nothing else - will work for some time, but sooner or later there'll be problems.
If your rabbit never had grass and hay, I would have his teeth checked. Those are alwasy growing, and need lots of chewing the right stuff to wear down properly.