What is the burden of proof on sexual assault?

(September 25, My son had consensual SEX with a girl he goes to college with. 3 days later (after she learns my son wasn't interested in a relationship) she filed a police report with campus police saying he RAPED her. An investigator questioned him to get his side of story and said he'd get back with him within a few weeks. Since then he has been suspended from his campus job pending this investigation. On 10/28 case was turned over to the DA office. This is a first for me and when I hear that it's being referred to the DA office and from there may go to the Grand Jury. I need guidance on how to proceed. Since he is of age, neither campus police nor the DA office won't give me any information. I live in Dallas county, but this happened in Hunt county. I need advice ASAP. He has to go to a university hearing next week, and we may need an attorney to be there as well.