I think this moment in the scene is overlooked because it’s rather subtle.
When they walk up to the GP Surgery, Simon is well behind Kieren, letting him lead their way (as will become typical).
Upon entering, Simon is looking at Kieren (first pic) for cues.
When Dr. Russo calls Kieren’s name, Simon’s attention switches to Dr. Russo and his expression hardens (second pic). Whatever Kieren thinks of this situation, Simon distrusts medical personnel (and with good reason, considering his horrific experiences with them, ESPECIALLY the ones involved with the care of PDS patients).
Simon’s reaction to Dr. Russo approaching Kieren is to move from behind Kieren to in front of Kieren, and stop moving (third pic, though it’s easier to see when they are in motion).
He doesn’t place his body between Kieren and Russo, he’s not blocking anyone from interacting or stopping Kieren from doing anything he wants to do… but he HAS placed himself in such a manner that Russo has to keep his distance from Kieren or else be weirdly in Simon’s personal space. Simon has also placed himself into a position to defend Kieren in an instant if needed, and his gaze is placed off to the side of Russo in such a way that suggests Russo doesn’t need to worry about him because he isn’t even paying attention to Russo. In fact, the rest of his body language suggests his mind’s attention is 100% on Russo.
I love, lovelovelove, this facet of Simon. He trusts Kieren to take care of himself, doesn’t restrict him from doing anything he wants to do, but Simon places himself in a position to step in if he’s needed.