I remember to have read as a child a story where an alien being, I guess being left behind by his friends, is wounded and has to hide, maybe in the attic of a house. His kind of aliens are being persecuted by the humans because they use blood as food that they drink directly from other organisms without killing them (vampire-like). A boy knows that the alien is hiding in the attic and though he is afraid, he helps him by letting him drink his blood. The alien is portrayed as a harmless, friendly creature. It is possible that they communicate telepathically, not sure.
Somewhat reminiscent of Hal Clement's "Assumption Unjustified" in which a benevolent blood drinking alien feeds on a young boy. Differences are that the aliens are unknown to the humans and the boy's donation is not voluntary.
I found the book again. The story is by Thomas N. Scortia, and the German translation I read is titled "Der Fremdling" ("the stranger", or "the alien"), but I wasn't able to find out the original title or publication year.
Edit based on the comments: the original title appears to be "Blood brother", published in 1974.