First, if you saved the video, upload it on Wi-Fi by going to the person in the back of the pokecenter on the right, and choose "View or upload battle videos". Go to pokecheck.org and go to the battle video section. Fill in the video number which you got when uploading. Now you can see the stats of both your and his pokemon.
If that doesn't work, try this:
Try to run some damage calcs: http://www.pokemonshowdown.com/damagecalc/ Fill in your EVs/IVs and for his you just have to fill in the highest possible (=legal) stats. If his attacks on you did more than possible, or yours on him did less than possible you can prove he's a hacker as well.