I guess that's because he's really a child since he don't remember his previous lives unless he reads the book of powers relating to that life. It's like his experience resets to 0 after rebirth.
but he remembered his previous life as a construction worker and he was an adult that.
Yep totally. So I've been thinking that since he lived a lot of life (the life transforms into a book of power that can be found in the library) and by reading this book he can actually remember. But most of it is just the memory and the power, not the actual experience itself so it doesn't really affect his mentality. I'm just saying this hahaha. But he shows maturity beyond his age from time to time. I might be overthinking it and it's just his whim to act child and enjoy life.
I love the fact that he still acts like a child. He's mature for a child but not the point like he's a man in a child's body.