I have seen Laker pant like that - back when he was building flight strength. When Rio was little he also would pant & hold his wings out when overheated. ... I have a photo of that somewhere ... here is a photo of him a bit overheated on my #3 reply, page1 Flying together.

You are right that the wing pose is in the same position - almost. The trembling only superficially looks like the overheated pose. It is a trick of the 'camera' that they look the same at all. If it was on video you could see him shivering, not panting. Also, the pose gets a more pronounced wing stance, than when he's hot.

Then there is the fact that he does it first thing in the morning & last thing in the night. He does fly about a bit but not more than Laker.

It will be interesting if he looses this nervous tick. It is very similar to a Grey's trembles + others have said that babies often use this expression to get parental attention.
Will Rio ever grow up?