Here's some unpopular advice, I say, don't worry about it. Who is he really cheating? Why does the cheater cheat? Basically, there has to be a reason why he feels the need to roll great numbers in an ultimately meaningless game situation. Does he cheat so he can lord it over the rest of you or does he cheat to make himself feel better about himself? Does he needs the ego boost of succeeding so badly that he has to cheat to achieve it? If the answers to these questions are yes and you don't want to deal with these underlying causes of his cheating they you either have to ignore the cheating or not associate with him.
We had a habitual cheater in our gaming group. He was going through some rough times in his personal life and one weekend started cheating at die rolls. When the party rogue goes from mildly useful in combat to slaughtering powerhouse, you notice. We discussed various methods for handling this (most of the passive aggressive). But in the end decided if this was how he needed to play the game to make it interesting to him, let him. Over time the cheating diminished a bit and as his life came back into normality so too did the cheating abate. Maybe he still cheats on the occasional die roll. I dont know.
Have the other players complained that he's cheating? They probably figured it out before you as DM did. Ask some of the other players if they suspect anything amiss and if they think you need to do anything about it. If the other players are not bothered by his cheating then there really isn't any harm caused by his cheating.
Now, since you posted here, his cheating must bother you. Ask yourself does his cheating affect your enjoyment of the game? Why? You have to determine if his cheating really affects you. As I say above, he may be cheating to bolster his own ego. Is someone whose ego is so fragile that they have to cheat at a meaningless RPG to feel good about themselves worth your feelings of, I don't know, resentment? anger? disappointment?
If you take his cheating as a personal affront, step back and think about that. Is he really doing it TO you? If you don't think so, then he is doing it for himself. Why? Since you care about his feelings enough not to just call BS on him immediately, how much of his emotional state are you willing to deal with?
OTOH, he could just be a jerk. In which case forget all I've said above just boot him. But if the cause of his cheating concerns you, perhaps you should just let it be.