boil it in hot water for a bit until it is soft then bite into it so it forms to your mouth
I'm going to assume he's done this, otherwise the Mouthguard doesn't work at all.
That's the basic instructions for a mouthguard.
If you try using one before you've done that then you're being a bit silly.
You might want to check that you can't get a better sized one for you.
I always had trouble with mouthguards.
Too much dribble. Had to try out a couple before I got a good one. And then the Rugby season ended and I never really needed one for other sports. :/
But, better too much dribble and a mouthguard than none and broken teeth.