<p>It really depends on how selective of a college he will be looking at. Only the most selective colleges are more likely to require or recommend 3 years of (the same) foreign language, while for the almost everywhere else 2 years is sufficient. </p>
<p>You can look up the requirements for a few colleges that you think are plausible ones for him to apply to. If he hates Spanish, my inclination would be to let him stop taking it in favor of a different subject that he would enjoy more.</p>