It looks as though your dog has seen something hostile in the gesture, and as an imature dog, he is still unsure of his position in the pack. this ties in with his dislike of your staring at him - dogs will 'stare down' each other to establish superiority. The dog that looks away concedes the higher status of the other, if neither will back down, a fight may follow. Your dog is keen to show his willingness to subjugate himself to his pack leader - you - and this is what his behaviour indicates. the more mature dog is secure in his pack position, and has no need to check for an superiority establishment by you - each of you know where you are in the pack hierarchy. If you avoid staring, or sticking out your tongue, and give him lots of love and affection, he will get over this issue, and settle down again.