The short answer is yes; he is always free to contact the police within the statutorily provided time limits, (these vary from state to state), and seek to file a criminal complaint. However, there are practical problems; specifically, he would obviously be asked by everyone involved as to why if he knew about the incident did he wait so long to get around to reporting it. Police and prosecutors are not very keen on letting themselves get involved in rather minor situations months down the road where it appears that they are being used by the complainant to "get back" at someone.
Yes they can always change their mind. The real question is will the police file a criminal complaint if the person changes their mind, Very often they will but they are not going to let the person go back and forth over whether they want charges filed. You should also know that it is not up to the person who was the victim whether or not to press charges it is up to the police. They will certainly take the persons position into account but I have seen charges filed and dropped numerous times in opposition to what the alleged victim wants.