Charles explains that both his continued relationship with them and his ability to speak about it is founded in the aliens themselves. According to Charles the cooperative relationship with the aliens and the military was in part contingent on the military's cooperation with their demands. Charles was one of their pets, and one of their demands was a continued relationship with Charles while he was employed with the military as well as the latitude to reveal the encounters.
I had the same initial doubt, especially since he released this as fiction first and later said it was true. Here is the way I dealt with it mentally. We can confirm he is who he says he is and worked where he said he worked. They have
testimony from several witnesses who worked in the same position as Charles or knew him who can at least corroborate some of the events in the stories as well as attest to UFO interaction at the base though no witnesses had any interaction with the ETs but Charles. So something ET related was very likely going on at this base, and so if his story is false it seems like it's more of a matter of how he was so deceived into experiencing all this.
When historians look at documents to determine the accuracy and truth value of the document they look for certain clues that can reveal the trust worthiness of the author. One such clue is if they reveal evidence that would either be counter to the authors purposes or beliefs. Hall is a catholic and a lot of the aliens information is in direct contrast to Christian belief. If he was motivated by money he might have had the aliens believe in something generic like "Christ consciousness". So there are little clues in his writings that might indicate he is not trying to deceive people.
I tried to see if he is lying by watching the Q@A after his speeches. He doesn't posture himself physically or mentally to prepare for answers. If this were all a theoretical construct of his (a lie) he would have to load up the entire theoretical world into his mind to answer questions so he doesn't contradict himself. But Charles just waits for the question then he calmly accesses his memory and responds based on his memory. He never appears to "make up" new content on the spot or become stressed over any of the questions. He is just very casual to confrontation, but also seems deeply disturbed by what he experienced.
So I am of the mind to think that at the very least he believes his story is true, and if it's not true something needs to account for the strange things happening at that base. If he is honest and his story is false I would be inclined to think the military was experimenting on him. But for me, given the two options, it seems to require less assumptions to believe him. That is how I came to view the whole thing, I don't consider it fact, but I consider it credible enough to begin working with when I think about things.