My dog keeps snarling and growling?

My dog name is Petey hes about 7 or 8 years old. He's a chihuahua. He's my best friend in the whole world. So, everytime my brother or dad trys to pet him, he snarls. But now, he has a habit of snarling, AND now biting. What my brother tried to do was squirt him with a water but that didn't seem to work it just got worse. And we are not doing a class for him since it would probally be too much money. Also, we tried putting him on the floor and pointing him out of the room but that seems not to work either. Have any ideas?