Ask if you can hold onto his dice between play sessions, and test them yourself if he consents, or lend him a set of dice to use while he plays at your table.
You can test whether a set of dice is weighted towards a specific result by using the test found in this question here. It will require a fair bit of epsom salt and a few hours of time, but if you're willing to do it, the test can quite easily determine a dies bias (as long as it isn't a D4 anyway).
This method will NOT determine certain kinds of fixed dice, such as shaved surfaces (where one side of the die is shaved off so that the die isn't a perfect cube, causing an imbalance towards one side because of its more rectangular shape), shaved angles (where certain angles are shaved to be larger than others, causing the die to land on the sides with the largest face more often), nor will it detect dice with liquid metal cores like gallium (which become liquid when heated up in a hand, and when the metal core becomes a solid again, it can cause the die to be weighted towards a specific result).
It will determine dice that have an improper weight as a result of air bubbles inside the dice, or more dense sections of the dice cooling as part of the manufacturing process. Most dice that are non-clear that are manufactured by Chessex fall into this category, you can find out more about that in the above question/answer.
If he consents to leave his dice for you to test
Test them. Determine their bias, and notate your results thoroughly. If it turns out that all of the dice are weighted towards high numbers, it might not necessarily be through fault of his own, he could've just bought some poorly balanced dice through Chessex. Explain to him that there are few dice that are perfect, and keep some of the epsom solution so you can show him how the dice were tested.
If he doesn't consent, and refuses to use other dice
There's a distinct possibility that he knows about their bias, or purchased the dice weighted on purpose, this is the most likely scenario which would indicate possible guilt, you can't really prove that he's lying to you, but his refusal to use dice that you have deemed fair might indicate that he is being intentionally manipulative at your table, and generally doesn't fit in very well with the social contract of most dungeons and dragons groups.
If this is the case, tell him that if he won't let you test his dice, that you'll be expelling him from the group until he will either use a set of dice you yourself have tested to be balanced, or are allowed to test his dice to determine their slant.
It isn't fair to your other players to allow cheating at your table, and if his dice are actually weighted in a way that would be fair, it could negatively impact the gaming experience of everyone at the table other than him. Nobody likes a Mary Sue, just like nobody likes a cheater.
There's a chance he's probably going to play the guilt card and attempt to guilt you into letting him stay even if this occurs, probably something like, "You have no proof yet you're still kicking me from your table, that's not fair to me."
If they do offer a rebuttal like this, just tell him that he is acting in a manner that confirms his guilt, and that if he had nothing to hide, he would have allowed his dice to be tested or to just be a cool dude and use someone else's dice for the remainder of the games.
If he doesn't consent to their testing, but will use other dice
Probably the easiest scenario that doesn't require science or thorough testing of dice honestly. If he will just use other dice, that completely takes the potentially loaded dice out of the equation, and allows for a complete diffused situation. No more bad dice = No more ridiculous rolls.
During the later years of the D&D Championship Series, a professional D&D competition; (Yes, they exist) everyone who entered and played was issued a set of dice, and were not allowed to use their own, this is to prevent the entry of biased rolls in the competition and ensure fairness for every participant.
You could pull a similar card with your table, buy as many sets of clear, see-through dice (with no air bubbles if possible), test them, and give everyone who sits at your table a set. They can use no other dice that you haven't given them. This takes away the fun of using your own personal set of dice, putting your dice in dice jail, or exiling them to the top of the dice tower, but insures fairness.
That's a huge trade off though. Many would consider the use of your own dice one of the staples of Dungeons and Dragons, and should only be done as a last resort in the event of suspected cheating.
