You should definitely read Matt Levine's coverage of the fiasco.
Quote from his newsletter:
> That contract does not allow Musk to walk away if it turns out that “spam/fake accounts” represent more than 5% of Twitter users. We discussed this last month, when Twitter admitted in a securities filing that it had (slightly) overestimated its daily active users for years. The merger agreement contains a provision that allows Musk to walk away if Twitter’s securities filings are wrong — and this 5% number is in its securities filings — but only if the inaccuracy would have a “Material Adverse Effect” on the company. (See Sections 4.6(a) and 7.2(b).) That is an incredibly high standard: Delaware courts have almost never found an MAE. An MAE has to be something that would “substantially threaten the overall earnings potential of the target in a durationally-significant manner,” the courts have said; there is a rule of thumb that an MAE requires a 40% decrease in long-term profitability. If it turned out that 6% or 20% or 50% of Twitter accounts are bots, that will be embarrassing and might even reduce Twitter’s future advertising revenue, but will it be an MAE?
There isn't a way for him to exit the deal – Twitter can compel him to go through with it based on the contract. The main question remains though: Will Twitter go through the arduous and potentially ruinous process of forcing him to honour his obligations? Is this a negotiating tactic on his behalf (Is he trying to get a better deal)? If he does want out, will Twitter compromise and take a settlement (they could ask for way more than oft-discussed $1B exit fee)?
This is Musk being Musk, and personally I hope if he does try to back out or renegotiate, he gets punished severely.