Yes, it is a VERY hard thing to do, to become, in four years, a native speaker fluent in a foreign language, even when you arrive at the age of 12. It is even harder when you live in a household where the home language is not English. It’s made even more difficult unless the teen prioritizes becoming totally fluent in the new language, by spending many hours reading in English, noting new vocabulary, and memorizing that new vocabulary. BTW, there are apps that one can use that present the new vocabulary words to you at ever decreasing frequency, according to whether you get them right or wrong, and present them to you just before you would have been expected to have forgotten them. My kids use them for foreign language vocab, for scientific and math equations, for anything very short that they need to memorize.
My comment was not chastisement of the OP’s son. It was just a statement of fact. The OP is considering whether they should have him do an extra year of high school. The question is, will it change anything? YES, it will, IF the young man is put into a good boarding school, where he would be living in English 24/7, and not in the home language outside of school hours, AND IF the young man is highly motivated, and willing to do the extensive reading of literature (on top of whatever he is assigned in English class) in his free time. I have a feeling that if he had been interested in reading in English for pleasure, he would have already been doing it. The family has to weigh the cost of two years of boarding school and the social issue of making a young man spend his 18th year in the restrictive environment of high school, against the possibility that he might gain better mastery of written English, and get into a more competitive college than he might otherwise have. Or he might not.
If they have plenty of money, and are willing to spend it on this, it might lead to him getting into a more competitive school. It might also lead to a frustrated and rebellious young man, who resents being kept in high school an extra year.
BTW, I have friends who arrived with two kids in sixth and seventh grade, speaking not a word of English, not even a European language. The school district would have kept them in “English language learner’s” track for three years. I met them around Xmas of their first year here. I told the parents to get them OUT of that “ELL” track immediately - just throw them into regular classes, plus honors math (they were smart, and good at math). The reasoning was that every day they sat in ELL, they were getting older and losing some of their brain plasticity to absorb language. Boy were those kids mad! They were furious that their parents had moved them, ELL was so easy, and now they had to WORK in school? After one semester in regular classes, they were both in all honors classes (seventh and eighth grades). They have both been straight A students in the highest AP/honors classes in high school, including AP English. By the time they were in 9th and 10th grade, you could not have told them from girls who had been raised here. According to the school district’s plan, they would have just been moving out of ELL into all regular level classes by that time.