How to tune a trumpet?

We have added a new instrument to our household. Aidan is now learning the trumpet at school. So far he seems interested, but the practising can be painful to listen to at this beginners' stage. Playing in a jazz band, a stage band, or even the fledgling school band, seems light years away.

I would love to know how to tune a trumpet. I have absolutely no idea when it comes to this sort of stuff. As a piano player and in a previous life, a xylophone player, I have never had to tune an instrument before. I don't think it sounds right but can anyone tell me if I have it right that an E on the trumpet is not actually the same as an E on the piano because a trumpet is a Bb instrument? When he is playing what he says is an E on the trumpet, it is sounding like a G on the piano....