Toy Poodle has just started to eat grass

Some dogs eat grass as a natural emetic when they have stomach pain. If it doesn't resolve quickly I would talk to your vet, especially if he also has diarrhoea. There have been a lot of very unpleasant canine stomach bugs around in the UK this year, or could he have eaten something he shouldn't? Is he drinking, even if not eating? Toy dogs can dehydrate quickly, as well as being prone to hypoglycaemia if they do not eat - both can be dangerous if not treated. Try him with dilute home made chicken stock - a little skinned chicken cooked in plenty of water, no salt, cooled to blood temperature. Dogs will often drink that when they refuse plain water. If he is not eating or drinking and seems lethargic I would not wait - call the 24 hour emergency vet immediately - but if he is bright and active then you should be safe waiting to see how he is in the morning. A day or two of a bland diet like chicken and rice may be a good idea until he feels better.