Not half as embarrassing as finding out he cooked the data.
You don’t have to ruin his reputation unless he decides to go public. Just have him resign “for personal reasons” or “to pursue other interests”. If he goes public, then you can just stick to the facts and let others infer that it was because his work was shoddy.
More damage has been to the climate change effort by people who went beyond the evidence than by those who deny it. I don’t know what the bottom line is but it sounds to me like it is important that your work be impeccable. This guy sounds like the weakest link in the chain. If I were in your position, I would have a serious meeting with the boss and explain that this could destroy your credibility and you gotta convince him of that.
You want one? I know one who would work cheap. 