Learn English – Word for a person who lives in the past

Perhaps self-absorbed

absorbed in one's own thoughts, activities, or interests

or egocentric

regarding everything only in relation to oneself; self-centered; selfish

or self-centered

concerned solely with one's own desires, needs, or interests

SUPPLEMENT

Also consider self-directed

(of persons) free from external control and constraint in e.g. action and judgment

Note that this term is often applied to someone who does attend to the needs of other but exerts a good deal of independence in doing so.

and autonomous

Not controlled by others or by outside forces; independent

A young person who demonstrates wisdom and maturity beyond their years is often called an old soul. It comes from the belief that some reincarnated souls retain a measure of the wisdom and character developed in previous incarnations. These days, even those who don't share in the actual belief find use for the phrase in normal conversation. I'd be perfectly comfortable referring to the boy you describe with this term.