Ask if you can keep his dice between game sessions, and test them yourself to see if he agrees, or lend him a set of dice to use while playing at your table.
You can test whether a dice set is weighted for a particular outcome by using the test given in this question. It will take a bit of Epsom salts and a few hours of time, but if you're ready, the test can pretty easily determine a die bias (as long as it's not a D4 anyway).
This method does NOT determine certain types of solid cubes, such as: For example, shaved surfaces (where one side of the cube is scraped off so that the cube is not a perfect cube and causes a side-to-side imbalance due to its more rectangular shape). shaved angles (with certain angles larger than others, causing the cube to land more frequently on the sides with the largest surface area), and cube with liquid metal cores such as gallium (which become fluid when heated in an oven) are not detected when the metal core solidifies again This can lead to the matrix being weighted towards a specific result.
Dice are determined that have a false weight due to air bubbles inside the cube, or denser sections of the cube that cool as part of the manufacturing process. Most unclear cubes made by Chessex fall into this category. Find out more in the question / answer above.
If he agrees to leave his dice for you to test
Test them. Determine your bias and write down your results thoroughly. If it turns out that all the dice are weighted towards high numbers, it may not be his own fault. He could have just bought some badly balanced dice over Chessex. Explain to him that there are only a few dice that are perfect, and keep part of the Epsom solution so you can show him how the dice were tested.
If he does not agree and refuses to use other dice
There is a clear possibility that he knows about their prejudices or has bought the deliberately weighted dice. This is the most likely scenario that indicates a possible guilt to prove that he lies to you, but his refusal to use dice that you think is fair may suggest that he deliberately manipulates you at your table and generally does not fit very well into the social contract of most dungeon and dragon groups.
If this is the case, tell him you will exclude him from the group if he does not let you test his dice until he either uses a set of dice that you yourself have tested as balanced or that is allowed to roll his dice test to determine their inclination.
It's not fair to allow your teammates to cheat at your table, and if your dice are actually weighted in a way that would be fair, it could negatively affect the playing experience of everyone else at the table. No one likes a Mary Sue like no one likes a scammer.
There's a chance he'll play the badge and try to get you to stay, even if that happens. Probably like, "You do not have any proof that you're throwing me off your table, that's not fair to me."
If they offer such a counter argument, just tell them that he acts in a way that confirms his guilt, and that if he had nothing to hide, he would have allowed his dice to be tested, or just a cool guy and For the rest of the games, use another player's dice.
If he does not agree with her tests but uses other dice
Probably the simplest scenario that honestly requires neither science nor thorough testing of the dice. If he uses only other cubes, the potentially charged cube is completely removed from the equation, resulting in a completely diffuse situation. No more bad dice = no ridiculous buns anymore.