Your son sounds like me (and the rest of my class of 'high achievers' I went to school with)... high IQ, nothing challenging or interesting enough in school to bother with.
Smart people often work out the real-world reaction to artifice like grades very early: what is an A worth in a store? Yeah. Next week, when that A's old news, how much is it worth (how much is it going to 'count' toward next year's grade)? Zilch. Outside the school system, does anyone actually care what grade anyone got? Hardly.
3.5 may not be much of a stretch for you lad -- but it does suggest a natural affinity for languages that he might like to think about. If he can get that good a grade by showing up (my experience throughout school), why bother putting any effort into it? Beyond 'above average' and already knowing he's smart, why put anything into something he's never going to get anything out of? It's a question HE needs to answer to his own satisfaction.
I figured out in grade 2 that teachers are not here to facilitate excellence or nurture curiosity... they're here to present the curriculum -- which often represents 100% of what they know about the subject they're teaching. It is a very rare teacher who thanks a small child for pointing out (through eager questioning) that the teacher doesn't have a clue what the answer is. So rare, in fact, that not once in my entire public school life did I ever meet ONE.
What is a small, eager, curious and bright child to do with their questions? Well... take them out of school, for a start.
'L. is not living up to her potential' was written on virtually every single one of my report cards... and my reaction to that by the time I was 13 was 'why the heck would I?' As in: what exactly would I get out of bothering? Higher meaningless grades? Yipee. More time wasted doing the same stupid problems I learned to do three years before they were in the curriculum? Hoop-jumping practice? Gee, no thanks, I think I'll use my brain otherwise.
THE reason I completed high school at all was because I was still under the hilarious misapprehension that it was necessary for anything. The reality (made plain by a whole generation of homeschoolers) is that even universities as prestigious as Harvard and Oxford *prefer* self-directed learners who haven't learned to value grades over learning, or how to jump through hoops instead of taking on a challenge.
20+ years later, the ONLY use I've ever had for my high school transcript, A's included, is having something to put in a safe deposit box.
Encourage your lad to do more of what he does well, and talk about the value of excellence (as a philosophy)... and decide how YOU feel about the contrived value of grades, so you can either let it go or make it genuinely important to you (as in: go get some of your own). Don't sell yourself short, either.