Let me start by saying,
I am not a doctor and I am only making suggestions based on my experience as a mom.
Does he do any stimming besides the rocking? Spinning, hand-flapping, anything like that?
Sometimes stimming is a reaction to sensory overload. So it may be that there is something new in his environment? Or may be that he is more aware of something now? It is calming. Think about it like this- there are lots of typical people that sort of absentmindedly will tap their fingers or shake a leg when they are bored or processing something or nervous. It provides something sensory that is needed.
I wouldn't try to stop it unless he is doing it in an inappropriate place where it's distracting to others. If he's verbal, you might be able to figure out why he's doing it- when it starts, ask him if he's excited, or thinking, or worried.
My son flaps when he gets lots of visual stimulation. America's Funniest Home Videos,
YouTube, marble mazes, that sort of thing.