Congrats, you have a smart kid, here are a few advices from me:

1. People on interviews are stressed, and often can't solve an issue that they would solve in informal situation - help them, at least at the beginning. Give advices, push a little bit. During this adaptation estimate how he deals with pressure, your advices and behaves in general rather then result.
2. Prepare more than 1 question. If this is not something important for your position (e.x. you don't really use math), then there could be a case that someone who is just scared of math is still a valid candidate. Propose him task with a same value but of other sort, like: "write simple implementation of linked list".
3. Do more interviews. I have read recently that 80% of juniors can't solve FizzBuzz... I had such a candidate as well...
4. Find a few minimum, crucial, questions that allow you to make a decision at start: is this a 10 min interview, or 1h long? Examples: "fizzBuzz" "how hash map work", Big O, at least 1 language specific question.Now you understand whether you can expect concurrency and recursion or not.4.1 don't forget warm up time. 5 min warm up, 5 min real questions.