How often should a puppy be eating or drinking?

I have a 9wk old puppy 3-5lbs, that I have had for three days now. Today was no different than the other two activity wise. He ate throughout the morning and afternoon, approximately 1/2 cup dry food. Each time he was taken outside he urinated(he was taken out quit a bit), he also moved his bowels 3-4 times today and the stool was formed. He seesms to be a grazer or needs reminding to eat. This evening he seemed disinterested in eating or drinking, he did however accept treats when he came in from going to the potty. He did sleep quit a bit today(I do know they require alot of sleep), but after napping in the evening and not eating during the evening, when he was put to bed in his crate he only wimpered briefly, which is unusual since the other two nights he cried for some time when put to bed. I am concerned with the lack of eating or drinking this evening. He did play a little this evening but seemed to only want to nap. How often should he be eating and drinking? Should I take him to the vet immediately?