Twin flames can trigger physical changes in each other. I do agree I remember reading about it a while ago so it does happen but is rare.
Some twins look younger during certain phases, or notice little shifts in features. Hair and eye color is probably the most talked about but maybe only because they would be the easiest to notice. The energetic exchange between TFs is so intense that it can influence how you perceive each other, and it can even influence actual physical appearance over time. I think that’s important. If this is a short-term thing, then it is more likely that your perception has changed. Third eye opening, heightened intuition, seeing beyond the physical. All tracks.
The closer you get to union, the more similar twins can appear - posture, expressions, even small physical traits start aligning. Seems to follow pretty well with the Kundalini Awakening aspect.
What you said about him seeming like different aspects, meek/serious vs immature/boyish, could be past-life bleedthrough. Those can overlay onto your current perception of them.