Hello. Thank you for coming to JustAnswer. I am Thoreau and I am sending this answer to you only a few minutes after you asked your question. It would have been a little quicker, but I wanted to provide some resources and citations for you.
I'm very sorry to hear about the situation. Mississippi has passed the Vulnerable Persons Act (VPA). The VPA makes exploitation of a vulnerable person a crime, and an elderly individual can constitute a vulnerable person if age has made her incapable of adequately protecting herself from exploitation. The Act can be found in Mississippi Code Section 43-47-5 (you can read it here: http://law.justia.com/codes/mississippi/2010/title
-43/47/43-47-5/). You can report exploitation to the Division of Division of Family and Children's Services and Adult Protective Services here: http://www.mdhs.state.ms.us/fcs_aps.html. The Division will then investigate the matter and he can be charged with a crime. Further, as you know and as you briefly mentioned, if your grandmother is incapable of managing her own assets, it's possible to petition to establish a conservatorship
according to Mississippi Code § 93-13-251. In that process, she would have the opportunity to show that a conservatorship is unnecessary and other relatives (including her son) would have the opportunity to try to show that they are best suited to be her conservators. The judge would then decide if she needs one and, if so, who would serve her interests bests.
I would again like to tell you how sorry I am that your family is going through that. It's very discouraging to see a family suffering from one member's greed, especially when it involves the victimization of a member of the family. I hope that things resolve in your grandmother's best interests and I wish you all the best.