Many children don’t like a musical instrument, dancing, or a sport because they are not very good at it. I wouldn’t encourage my children to avoid these things because they are not good at them! Several years of club attendance, encouragement and careful, considerate support and many go on to thrive – and very much enjoy things they once disliked because they weren’t good at them. Know the difference and make sure your son knows the difference.
There's a world of difference between "encouraging" a (say) 7yo to pick up a musical instrument while you (more or less) stand over them and make them stick at it until they get good enough at it to enjoy it, and making a potentially stroppy and hormonal 13-14yo choose something which will occupy the next 2 years and where you will be absent and largely oblivious to what's happening for 99% of the time. "Know which battles are worth fighting" is another worthwhile and rather obvious sentence to apply here.
It's why I chose to ask whether he enjoys French. If he does (or is prepared to work at it because it may open up opportunities he wants to pursue) then by all means choose French. But if he doesn't/isn't, and especially if it means he has to drop something he does like, then it may all be a waste of time.
It's not as simple as cooing "ooh yes, a language is always a good idea" as some here are prone to do. Not everyone enjoys languages, some people have no talent for them and, to be brutally honest, most people have little or no use for them. Nor is a GCSE in any way a prerequisite to learning French or a love of French culture and life (quite the reverse in most cases, I suspect!). The poster is best placed to know whereabouts in all this his/her own child lies.