Getting the diagnosis and the treatment to go along with it is a Godsend! I know it seems like a lot to deal with now but you, your child, and your family will be so much better off. I am glad to hear that the diagnosis came early and you are addressing it head on instead of overlooking it.
I always knew something was off with my oldest (now 5 1/2). She's as sharp as a tack but life is a struggle for her. She just doesn't behave like most children her age. ADHD was suggested but here a diagnosis for that can't be made until age 6 with certainty. Knowing she can keep at task (doing a puzzle, artwork, math problems into the thousands) for over an hour rules out ADHD in my mind but the child can not sit still for the life of her. She bounces off the walls and other kids, can't keep things out of her mouth, and does down right bizzare things. She doesn't feel pain unless it is super severe yet the *** of a feather the end of a feather will set her off in a panic. After over a year of going to behavioral therapists and being told there was nothing wrong ... she just needed time to develop, she was recently diagnosed with sensory processing disorder (SPD) or sometimes it is referred to as sensory integration. The gravity center in her brain never developed as it should by 18 months old. She has no sense of feeling centered in the world so to her life is chaos and she is trying to feel grounded.
We go to occupational therapy once a week. We do at home treatment daily (work with exercize balls, motor skills, rocking, brushing, etc.) The therapist will work with you so that you can make things easier for your child so he can adapt to the world at an easier pace.
I've gotten a lot of help from Kirk Martin at Celebrate Calm (www.celebratecalm.com). He addresses ADHD, SPD, and a variety of other challenges that children and families face. Just signing up for the free email newsletter is worth the visit to the website. The thing I love about his perspective is that he can help the parents focus on the positive nature of the child rather than the challenging parts. He also gives a fantastic undersanding of what is actually going on in the brains of our child that is easy to understand. Another great resours is the SPD Foundation based in Colorado. They deal with SPD and autism (as they so often go hand in hand).
You are not alone. The sooner your child gets help the better the results you'll see. My prayers will be with you!