He just seems to think that if he doesn't have a good chance of getting it right, he won't bother. Weird.... and frustrating!
Frustrating but normal. How often do you put your school girl French to use in France...too scared of getting it wrong? The have a go mentality is not natural for many many people, particularly those that are more reserved or more scared of the implications of error. I am not suggest you are telling him off, but if he puts something daft do you raise a higher eyebrow than a missed question? I know I do, which is why a teacher I will never be!
That is such a good point and something I need to work on (likewise I'm just not teacher material). I'll start building up his confidence to make mistakes, which does feel counterintuitive given what we're doing, but it's definitely worth a try. I'll try to pay equal credence to missed answers and incorrect ones as well. PS, my attempts at speaking French usually result in them saying "it's OK, it's easier if I speak English"!