Can he see his own reflection ?
My 2 beardies regularly headnod when there is no other dragon in sight, and my girl will arm wave sometimes even though she cant see our boy dragon.
Just a beardie quirk.
Try armwaving just like a beardie back and see what happens. I get the most puzzled looks back when I do this to our girl , maybe my way of armwaving is using strange words or bad grammar (in beardie body language).
It usually stops her in her tracks and she'll stare at me for several minutes before dismissing me.