Only a handful of highly selective colleges (e.g., Harvard, Princeton, Johns Hopkins …) ask for four years of a foreign language. So, if your son's high school counts his 8th grade year as an official year (and the majority do), then, technically, he is okay as is at most colleges.
BUT (and you could probably tell that a “BUT" was imminent!), applying to the pickier colleges (even those that ask for only three years of foreign language), with a year of middle school language and two in high school, could put your son at a slight disadvantage when compared to his “competitor" candidates who may have taken the same language for four or five years and through the Advanced Placement level. While it won't be an automatic deal-breaker for your son if he stops now, he won't be doing himself any favors either.