I don't think there would be legal repercussions if he insisted the letter was fake. Aside from defamation being tricky to prove (just ask Evans!), there is also the issue of damages and whether it's worth it to pursue legal action when you stand to gain little or nothing (just ask Evans!). The issue, for him, is that claiming it is fake could blow up in his face, so he will wait and see if it goes away. If not, he might claim it's fake, knowing that it becomes another potential bomb that can go off at any time. His other options are:
1. Admit it's real and that it isn't anyone's business but his and his wife's. I think that this is true! But also irrelevant. Remember, he's not trying to convince us of anything. He's trying to convince potential (and current) supporters that he deserves their money. Gruffly dismissing them and asking that they mind their own business might mean that they decide to spend their money somewhere else.
2. Admit it's real and try to justify what he wrote. I can imagine a scenario where he tries this, because he's so certain that he's the smartest man in the room. I hope he does go this route. The entertainment value would be immeasurable. So would the self-inflicted damage.
Other than that, he can continue to pretend it doesn't exist and try to dismiss it as part of the "defamation campaign" against him. We'll have to see how that works out.