Single word for a "like brothers" relationship
Solution 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)