My son does bruise very easily, but me and my mom do too so...
I did ask about it at his clinic visit but they had just doen his yearly vitamin levels and everything was fine. But he's 2 and very reckless (jumping off furniture, constantly running and tripping over things) and all the bruises are little ones on his knees, shins, and lower arms, so I guess that's what its from. But, when he's sick or having allergy problems he gets those dark circles under his eyes that almost looks like shiners.
But if he just woke up in the morning all of a sudden with bruises, I'd call the doctor. See how his vitamin levels are doing and make sure it's not from a med he's on. Good luck!