there is so much layered into this single passage. most of this do not make sense without the context that is provided by the story prior to this point. up until this point, we learn that Beto is gay, that the narrator has a prior sexual experience with Beto, and that the two are best friends.
what readers are provided with here are very vivid images of how the narrator begins viewing his friendship with Beto.
first, we have Beto with “…his hands bracing against [the narrator’s] abdomen and thighs.” while not made explicit, there is a level of sexual connotation which is made stronger by the previous sexual experience between the narrator and Beto. we can safely assume that something sexual is happening in this scene, when later on we read the line “After I was done, he laid his head in my lap.”
second, there is the rather warm and pleasant description of being “…caught somewhere in between [being asleep or awake], rocked slowly back and forth the way surf holds junk against the shore, rolling it over and over.” while this can be read as being laced with sexual imagery (rocked slowly back and forth, rolling it over and over), there is also the focus of the narrator comparing himself to junk. the language used here is so specific – seaweed could have been used, or some sort of aquatic lifeform. because of this, we can assume that the language used here is intentional. this provides insight into the narrator’s mentality about himself as a person – he is not going anywhere, literally stuck like junk being moved by the ocean, alternating between being rolled and being rocked.
lastly, we need to explore the actual friendship between Beto and the narrator. why is it important for us to know that “In three weeks [Beto] was leaving”? i think there is an underlying tension beyond the sexual advances from Beto that causes resentment in the narrator. the narrator’s mother is someone that pressures him into being friends with Beto, acknowledging Beto as someone successful and going somewhere since he is going off to college, whereas the narrator still has a year of high school to go with no plan in mind for after graduation. it’s very plausible that this would have caused envy or spite within the narrator, gradually corrupting the friendship between the two, and adding on to the (maybe unwanted?) sexual advances, we can read “In three weeks he was leaving” as something the narrator is happy about.