At that age you may be dealing with his possible interest in challenging you. Although as others have stated, it might be a medical issue but you just had him checked. As some males mature and the hormones start flying and they will test their owners.
I have one of those males and he has been trying to test me since he was 8 months old. He loves being with me and is great with others, including neutral dogs. I generally try to redirect him into something he loves to do, which in his case is playing ball. So if I see he is going there I immediately give him a command and reward with a ball or tell him to find the ball. It works everytime. I have some other approaches, but would suggest that you consider a behaviorist to help. I have been thru this before and in time it subsides as they grow out of that phase and understand their rank position, but it needs to be addressed in a firm positive way, ignoring it certain times and redirecting other times.