No, that's literally the most realistic thing happening, hands down. He's literally a child again. I don't care if he has a previous life's memories. He does NOT have his previous life's brain. It wouldn't even fit into his current skull, as-is.
A brain is a complex bio-chemical computer. Memories or not, he has child's far less developed brain, that operates exactly as you'd expect a child's brain to operate. What you seem to expect is far less realistic than any magic system in any series written.
Now being stupid (even for his age or not), that's another question. Having read only chapter 1, I'll see. It didn't seem like he truly had a well rounded adult life either, in a very different world, so I don't expect his experiences to be very useful in this one.
I don't know why you would respond to an almost 3 year old comment trying to argue about such a small detail, maybe to engage in a conversation about science? But your lack of willingness to accept a different opinion ("I don't care if he has previous life's memories) and being snarky in general, it makes me think that you just wanted to larp as a smart person on a three year old thread. But since I'm actually educated on Neuroscience (and was kind of in the mood of writing) so I will play.
Your argument that an adult brain wouldn't fit into his current skull is right, that is if you're going on the basis on pure science and the assumption that the brain works like an SD card where you copy just the memories and paste it onto another brain. Since you want strict science, so let's stick to it, right?
Memories are physical patterns, not separate data files. You cannot transfer the memory of an adult without physically recreating the exact neural configuration that encodes it. That configuration includes not just a single isolated engram, but the whole associative network that gives the memory context and meaning, which includes connections to emotional centers like the amygdala, and to the prefrontal areas that help you reflect on it. So if you magically imprinted episodic memories of an adult onto a child’s brain, you would necessarily be rewiring large portions of that brain to resemble the adult brain’s connectivity. The distinction between memories and the brain collapses.
You talk as if his previous life's memories are a lightweight possession separate from the brain. You are trying to have it both ways. He has his adult memories, but he is also a child in the PFC. Of course, this magical and dualistic splitting of the mind and body that neuroscience DEFINITELY doesn't permit, is not unrealistic at all! What's more unrealistic is expecting some degree of adult emotional expression and the presence of mature synaptic patterns, especially between prefrontal and limbic areas, giving him faster emotional recovery after outbursts, conscious access to coping strengths and a higher baseline of self-awareness. Now THAT is so unrealistic! (your words, not mine)
If you want to be a 100% strict and nitpicky about the biology, then the whole argument is gone. If you want to be 50% nitpicky, the whole argument is gone again, because we already discovered how the brain works, congratulations on that part. And if you want to be 10% nitpicky, and cherry pick the Science that conveniently only helps your argument, then yeah you would be right.
So which one is more unrealistic?
If we want to strictly go on the path of realism, it's more realistic to be able to accept that somehow his fully adult brain was transferred to his reincarnated body, through some rare case of biological miracle. You're trying to sound sooo scientifically rigorous when your argument is the least realistic, medically outrageous, and philosophically incompatible with modern cognitive science.
Since we can't cherry pick Science because that will only prop up one side of the argument. And we can't be fully strict on it as well because that makes the whole argument bullshit. The only option is to argue about which one is more realistic.
A scenario where he carries the adult brain. Physically impossible in practice but conceptually coherent with materialism. The mind is the brain, so move the brain, move the mind.
Your scenario, which requires memories to exist as separable, non-physical information. It's also impossible, in a different and deeper way, because it violates the fundamental principle of neuroscience that mind and brain are not separate substances. Your scenario is conceptually impossible even as a thought experiment without resorting to dualism. It contradicts how we know brains encode anything.
You're rejecting the scenario in which he has an adult brain, calling it insanely unrealistic (which is true, but irrelevant because I never argued for a literal brain transplant) BUT SIMULTANEOUSLY insisting that the scenario which you're arguing in favor of is what's happening, and then using biological realism to demand 100% child-like emotional behavior. Your scenario is already a magical, non-biological premise. You don’t get to smuggle in a miracle "memories without physical brain changes!" and then draw hard biological boundaries around everything else as if no miracle occurred. It's like saying "I accept that a wizard turned him into a frog, but because he’s a frog he must only eat flies. There is no way he retains his human taste for pizza!"
Once the fundamental rules are broken, you can’t selectively invoke them to prohibit related consequences.
And that my friend, is the:
Special Pleading Fallacy
Have a great day.