When should I be worried about my guinea pig?

I just got a guinea pig yesterday. I'm a little concerned because I haven't seen him eat or drink anything. I held him a lot yesterday, but now I'm trying to let him relax in the cage so he'll eat. I gave him kale and a carrot, but he hasn't touched either. He might have eaten some of his food overnight, but I can't tell. Is he just adjusting still? When should I be worried?