Therapists are generally not medical doctors. Psychiatrists are medical doctors. In all but one or two states, therapists cannot prescribe medication, only medical doctors can do that.
Therapist generally use methods aimed at changing behavior which do not involve medication. Sometimes they work closely with a psychiatrist, exchanging information and opinions, who prescribes medication. But therapists themselves work with behavioral modification aspect of treatment.
Keep in mind that there are rarely magic pills that make everything ok. All the medications prescribed for behavioral disorders have potentially serious side effects, including worsening behavior. Often they work for the behaviors for which they are prescribed but they cause others, sometimes worse ones.
If indeed you son does have ADHD, you should know that medication is not the only treatment available. There are behavior modification methods of treatment which are what you and he might learn about from a therapist. (Naturally with a child that young, the parent would be a big part of any behavioral modification treatment plan.) Also, none of the drugs used to treat ADHD are approved for children under 6 years old. That doesn't mean that doctors don't prescribed them for children younger, just that the studies, including long term studies, haven't been done that show the drugs to be safe and effective.