What are the precise reasons he is suggesting ADHD?
As his mom, do you notice certain triggers that make him more "hyper" or exhibit any of the other characteristics the therapist is concened about? Loud noises, crowds, certain foods?
Just to provide some perspective -- they have proven definitively that certain common food additives (artificial dyes and flavoring I believe) cause hyperactivity in many children.
As a mother of three, who has been involved in schools and with a program called Handle ( http://www.handle.org ), that helps many kids (and adults) with assorted issues, I have seen many children whose parents discovered sensory issues were involved, and advocated strongly for their children. (Read "The Out of Sync Child" for info.) I feel passionately about this because I have seen so many kids put on drugs as a first resort, although school was the only place they exhibited symptoms; and I have seen many avoid these strong medications, as well as harsh disciplinary actions, because modifications were made in their environment to meet their needs. Many doctors, therapists, and schools are still unaware of the role these irregularities can play. When you understand how the body's vestibular, auditory, and other systems work (and how they can be impaired), some of the so-called ADHD behaviors make perfect sense.
As with autism, there are many theories about why sensory "disorders" are on the rise--partly they are now recognized.. I personally feel that there are more offenders in the environment, from toxins to loud noises to an onslaught of electronici media. Also, there is more pressure at school with increased testing and standards, and less tolerance for the squirmy or active or disruptive child who may just be trying to cope.
Good luck, and follow you gut, you are the one who knows your child best. He will probably show you what may be going on.
PS - I would also look at http://www.hoagiesgifted.com and read the characteristics of "gifted" kids, e.g. heightened reactions, sensitivity, etc -- hm, sure sounds a lot like ADHD.