I too have been through this with my now 9 year old daughter. She is still worrying a bit, but it's not as bad as it was 2 years ago. It's not an age thing, but a "middle child syndrome." Even if your son isn't the middle child, he may feel left out because you and your husband are so busy with everything you have on your plates.
It really helped us to be sympathetic to her feelings, but make fools of ourselves in front of her. My huband and I did this as often as we could so that she could see that we are not perfect. When we realized she may be crying out for our attention, we also made a point to talk with her about things that maybe the other 2 sisters didn't know about. We confide in the other 2 also, but this goes in cycles. We don't confide in each of them in one day, that way each child feels like they've had special time with us. Try talking about small family problems and ask what he thinks-make his opinion count that way he feels a sense of belonging.
Please don't take this the wrong way and get angry at what I am about to say, I am playing "devil's advocate" here to try to help. As an outsider, when I read your "about me" paragraph, I see you put special emphasis on your special needs children, but not the other 2. It is possible that your son feels left out of the love/attention he is craving, and he is acting the way he does to get attention from you. I am not sure if that is part of it, but try giving him some special time with just him, even if it's only 10 minutes a day.
Good luck. I know how frustrating this can be.