JiJi's tummy ache

My kittie has been showing less interest in his food lately. I don't know if it is his new food or his plastic bowl, but he eats very little and begs like he is starving when we eat, even if it's a peanut butter sandwich. He spent very little time outside this morning and he is usually a prowler. I'm concerned, this morning a saw him eating grass and found some vommit on the steps, but now he's throwing up the water he just drank. I'll get him straight to the vet in the morning, but what can i do to help tonight.