Hi everyone, I need some advice

Coco's been acting strange the past 2 days. Just suddenly he's quiet, always sleeping, not drinking or eating as much and surprisingly refuses to eat his treats:S:S He's never done this before...he's only 1 year old. Could he be sick? Do dogs even get sick? I checked him all over and nothing seems wrong, he's not vomiting and no signs of diarrhea it's just his personality seems to have changed like he's completely opposite to what he usually is? Has this happened before? I hope someone may be familiar with this situation....
Thanks

-Maria & Coco =))