I read this book as a teen in the early 1970s. It had a typical teen book dust jacket design from the 70s: pen/watercolor painting of profile of boy with blue hair (I think) and I have a memory that it was called “The Blue Boy” or something similar but all searches have turned up nothing appropriate. 
Anyway, this memory is decades old, but what I recall is that this teen boy (probably orphan? no parents present in the story that I can recall) is somehow involved with a gang of bad people. Perhaps boy has magical abilities that they are taking advantage of for ill-gotten gains? Perhaps boy has blue hair as a gimmick? Maybe there is no blue hair but I swear there was. In any event, boy decides to escape from his situation and is pursued by bad guys who want to recapture? kill? otherwise cause problems for him. Boy is on the lam and has an unexpected mysterious ally who brings him food and perhaps finds him safe spaces to live. In my memory this ally is a talking mongoose, but crikey, how does this narrative even make sense? The boy is astonished to get ripe mangoes when in his world (the world of our narrative), mangoes are still green and unripe; months and months away from ripeness. It turns out this ally is from a parallel world, and the book ends with the ally helping our boy escape his pursuers by moving to the parallel universe. In my memory, book ends with boy on a sunrise-shining beach in this parallel world. 
I read it in English, but nothing about the story seems to be set in North America (mongoose and mangoes, or at least mangoes even if the mongoose is a figment of my imagination or faulty memory).