This is an old post - I found it during a search. My 5 year old son does this EXACT same thing. Hoping you are still getting notified on this post - and wanted to ask if your son is still doing it - or if you ever figured out what it was??
No he just does it on his own and does it all the time. Like, I'll be talking to him and he'll just start finger drawing in the air and then ask him what he's drawing and he'll say, a diamond...a train...a trapazoid...a nose...i mean it's all different and nothing in particular...
Is he copying another child? ask his teacher if any other children do it, or anyone at home or FamilyAsk him Why he does it .