My daughter has constant problems maintaining normal Ferritin levels but she also has had extremely low B12 and has a few medical conditions that may play into all of this. She always only had very mild tics, more anxiety,ocd issues. I don't remember what my son's levels were. It wasn't something they were looking at way back then and any blood tests that were done routinely for him were Strep. related (GABHS). Both of them have immune system related issues. There also appears to be a link to Restless Leg Syndrome (yet another condition relating to dopamine dysfunction) which I have, although that's been very stable for me for a long time now thankfully.

EDITED to add: Katy, I'm just curious, but does your son have a gluten intolerance?