This happened to my niece on Concerta...she ended up having to change meds.
Concerta has the potential to cause tic like spasms that in some cases can be long term. It would be worthwhile to see if they could refer you to a movement disorders specialst to rule out this concern. You can find out more on the medication website or PDR (Physician's Desk Reference).
This is going to take some research. None of my immediate searches of info was helpful. I will get back to you.
How old is your son? When do these things happen? How often do they happen? Perhaps the most important question is when do they happen - so be as specific as possible.
Perhaps he needs his dosage reduced. Children's needs change.