Hope he is ok. Real grass, as opposed to that kitty stuff, shouldn't be a problem I don't think. When mine had a cat run, they used to nibble every day and then chuck up (never outside; always on the carpet!).
I had to remove that pot grown kitty grass from Max as it made him vomit a lot including weird colours. Could you try him away from grass for a few days to see if he settles? Try not to worry (I know, imposs).