Single word for a “like brothers” relationship

Question

I'm looking for a single word that describes the younger man in this relationship:

A cop in his thirties befriends a seventeen-year-old kid he picked up on the street. He sees a younger version of himself in the boy, and the boy doesn't have a father figure in his life (he's being raised by a single mother), so the cop takes him under his wing. They develop a very tight friendship and have a relationship somewhere between father-son and siblings, though probably closer to the older-younger brother relationship.

I need something stronger than "friends," but since they're not actually brothers, I can't use that either. Similar to "protege," but with a less official connotation (and he's not training the younger one to become a cop).

Answer 1

I think you can describe the relationship as brotherly or fraternal suggesting that they are like brothers:

• of, resembling, or suitable to a brother, esp in showing loyalty and affection; fraternal. (Collins)

Given the context described and the fraternal relationship, the younger man could be referred to as the policeman's little brother:

• A close male friend; (AHD)

Answer 2

The word sounds a bit silly to my ear, but the younger person can be called a mentee, the counterpart to a mentor.

a person who is advised, trained, or counselled by a mentor. (Oxford Dictionaries)

Over the course of five years this evolved to a more casual adolescent to adult-model relationship in which the preservice teacher helped their mentee set goals, shared college experiences, and chaperoned college campus tours.

-Radcliffe, Richard and Bos, Beth, "Mentoring approaches to create a college-going culture for at-risk secondary level students." American Secondary Education. 2011, Vol. 39 Issue 3, p86-107.

In our context the older person might be called a youth mentor, to emphasize this particular sense, rather than the sense that entails professional training. But I've never heard an analogue for mentee that eliminates the corresponding ambiguity. (One can find about 1000 hits for youth mentee on Google, but this sounds awfully stilted to me, and I'd anyway avoid the term because it's potentially confusing enough to demand more explanation.)

The organization Big Brothers Big Sisters (the fraternal metaphor is explicit here), which arranges this sort of mentorship between adult volunteers and children, uses the peculiar term Littles, but I've never heard this usage elsewhere, and again without more context it would surely cause confusion.

International research has shown that positive relationships between Littles and their Bigs have a direct and measurable impact on children's lives and of the community as a whole.

http://www.bigbrothersbigsisters.org.au/about-us/our-impact/