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.
If your son won't be aiming for the more selective colleges, then he's fine with his three years. But if he's likely to apply to highly selective colleges (the ones that admit about a third of their applicants or fewer) that expect three or even four years of foreign language, then urge him to take another year unless …
1. He feels that it would be truly heinous to continue
2. He will be replacing the language with some challenging classes in fields he prefers, especially math and science.
Another option would be for him to start a new language and continue it for two years. While the choosiest colleges claim that they'd rather see four or more years of the same language rather than a 2+2 or 3+2 combination, your son might enjoy gaining at least a little competency in another language, especially if it's one that's very different than the language he already studied, and this will “look better" on applications than what his current transcript shows.
If you can afford it, your son might also consider a summer program in a country where his language is spoken (assuming it's not Latin). 😉 Although a summer program usually isn't comparable to a year of language study at school, and he probably won't get academic credit for it (though depending on the rigor of the program he chooses, he might) this could help your son improve his skills and even enjoy them. It would also show colleges that he has a little more interest and experience in this language than what his transcript will suggest.
As a parent myself, I know it's important to pick my battles. If my son seemed adamant that a language class was ruining his life or keeping him from following his true passions, I'd probably let him quit. But if the worst I got was a small groan, then I'd most likely insist that he continue for one more year.
By the way, you can find the foreign language requirements (or recommendations … since sometimes there's a little wiggle room) on each college's admissions Web pages. The College Board Web site also includes this information. (Go to https://www.collegeboard.org/ and type in the college's name, click on the “Applying" tab on the left-hand menu and then on the “Academics & GPA Tab.") If your son has already expressed some interest in any particular colleges, you might want to look them up with him and show him how much foreign language each one demands. Depending on what he finds, he may just decide to continue his language study without further prodding.