Oh, I forgot to mention that Ashley's book has a great section on 504 plans.
Definitely seeing a specialist is a good idea.
Since about 25 to 30% of kids with ADHD have anxiety as a co-disorder ("The ADD/ ADHD Answer book," by Susan Ashley, p53), its possible that this is a manifestation of that.
You didn't mention if he was on any medication. Sometimes when the meds wear off, this type behavior can get worse. So whether he is on of off meds, do make sure that you start writing down when and where this happens so the specialist will have this information. Also, if he has recently started or changed meds is important.
In terms of his behavior. Once again, it partially depends on if he is on medication. It also has a lot to do with how structured his teacher is. Being the CL on the ADHD forum, I do have lots of links to some very good resources that you might find helpful. I do want to stress that it is very important that you are aware of what ADD is and how it can affect a child! And if he does not have a 504 plan yet, it might be a good idea to get one in place before middle school.
Did your son have difficulty with speech when younger?
This is something that many an occupational therapist will work on. Typically they will try to replace that action with another. My own son doesn't lick his hand but for a long time tried to chew his sleeve or anything IN his hand. A hand fidget helps.
But I'm curious about when he was diagnosed with add and how his speech was when younger?
I don't know if anyone in this forum could give you competent answers without knowing more about your situation. The appointment with the specialist is a step in the right direction. Unless you find out why your son licks his entire hand, a resolution to this issue may be difficult. You probably already have done this, but have you asked your son why he does this? If so, what was his response? If he truly doesn't know why he engages in this behavior, a specialist would definitely be of benefit.