Thank you for contacting JustAnswer.
I am sorry to hear about the problems you are experiencing with your son. It sounds as if you are trying to fix the relationship between your son and his grandmother. This might be much better handled if your mother was willing to have a conversation with her grandson and apologize and explain what she meant. Hearing this from his grandmother may feel very different from hearing it from you. You can encourage your mother to work on offering your son unconditional love and perhaps finding something that she may be able to do with him that he might enjoy.
In the meantime, you can focus on setting limits for your son so that negative behaviors are not rewarded with attention but are given consequences. Keep in mind that any behavior that gets attention is likely to continue happening. It has been called the law of the soggy potato chip in that if a child thinks that he has a choice between a soggy potato chip or no chip at all, he will choose the soggy chip. If your son feels that he has the choice between negative attention or no attention at all, he will choose the negative attention and so he will act out until he gets it. The only way for this pattern to stop is to begin to catch the good behaviors and reward them with attention, and to calmly and matter of factly give consequences for the negative behaviors with as little attention as possible. A very good book on this subject is Win the Whining War & Other Skirmishes: A Family Peace Plan by Cynthia Whitham MSW. The more consistent you become with this positive parenting, the more secure your son will begin to feel and the more his behavior should improve. I hope this answer is helpful. Please let me know if I can clarify further.