Hi,
I'm not an expert, but my son has Tourette's Syndrome and I've seen a lot of tics...So, subtle head jerking is a very common tic. Biting his shirt sounds less like a tic or, if it were a tic, I would expect it to appear after a lot of other more minor tics have occurred for a fair amount of time. Perhaps he is anxious and the shirt biting is a way to relieve the anxiety. There could be more than one thing going on here. Did both behaviors start at the same time?
Tics usually begin in the face and head, e.g. blinking, grimacing, licking lips, head shaking. They often then progress to other parts of the body, e.g. arm jerking, kicking, etc. Vocal tics, when they appear, usually come after motor tics have started.
Has your son recently switched medications or had an increase in his Concerta? Stimulants often can lead to tics and/or increase anxiety.
A call to you doctor is a good idea. As you already know, tics do just have to run their course. Our difficult child doesn't like to talk about his tics. Sometimes I check in with him if I see a new tic that looks like it might hurt or be uncomfortable for him. Has your difficult child mentioned either the head jerking or the biting? Does he seem bothered by either?
My son started having tics when he was three and a half, but we didn't really talk to him about them until he was almost six (when he was diagnosed with Tourette's Syndrome). At that point, he seemed bothered by some of his tics and it seemed like the time to tell him what a tic is and that he has something called Tourette Syndrome. We approached it in a very low-key way and we talked about my brother, who had a tic disorder as a child.
Good luck.