Obsessive compulsive disorder (OCD) is a type of anxiety
disorder in which affected individuals experience intrusive thoughts and compulsions that they often must act upon, usually with repetitive motions or behaviors, in order to relieve the anxiety. The disorder is very common, even in children. In children up to 1% of children may have obsessive compulsive disorder, and it is usually diagnosed in the age range that your son is currently in.
It certain does sound that your son has a compulsive stereotyped behavior with relation to turning door knobs (touching the door knob three times). However, this alone would not make a diagnosis of obsessive compulsive disorder. He would also have to be experiencing anxiety and thoughts and compulsions that were significantly interfering with his social functioning (friends, grades at school, persistent fear or anxiety, etc).
If your son is experiencing some of these latter symptoms, then you should definitely have him evaluated for obsessive compulsive disorder. You could start by going to see your primary pediatrician
who can perform a preliminary assessment and then decide if referral to a child psychiatrist
is necessary. If such a referral is necessary, your pediatrician can recommend a good child psychiatrist in your area.