It's nice that he inspired you so much; he sounds like a superlative teacher.
That you got such great exam results proves that you have it in you to succeed later on. Teachers can help motivate you, yes, but enjoying French anyway will see you sail through A-level, should you choose to take it.
I studied French at A-level and it was pure thirst for the subject, the culture of the country and the intricacies of the language that saw students get their As. Indeed, my A-grade classmates have almost all gone on to study their language/languages at Uni.
In my opinion, you therefore have no need to stay in touch with him on an academic level. I don't want to offend you (at all) but have you considered that you might have a little crush on him? If yes, if no; I reckon it would be healthier to let him go when he does leave.