J.D.
Hi Becky,
You didn't clarify in your note how you think this going to affect your son. Does sugar cause him to have problems? If so, then you are right to be very concerned about him eating candy. If not, then I am not sure that taking a stance that he should not ever be given candy by his grandma is a realistic approach. It's great for all of your family that you want to be healthy at home and are setting a good example for your son, but didn't you ever get candy as a kid? Do you think it caused problems for you? I think you can have some sugar or "junk food" in moderation without it causing ill effects on your health. Maybe you and your mom can come to some understanding about how much and what type of sweets she can give him.
However, your concern about how your parents behave towards you as a parent seems to be a big concern for you. It appears from your note that you and your parents are not on the same page with communication and understanding. Has your mom always been difficult or is this a new development? Have you talked to your dad about your concerns about your mom? When you say unhealthy influences what do you mean? She may have some kind of health issue or it may just be how she is reflecting her problems with her relationship with you ie, she is mad at you about what she may perceive as your interference in her relationship with her grandson and she is not handling it well. There could be a lot of things going on here as far as the reasons for everyone's behavior but before you cut your parents out of your son's life see if you can work on your communication with them first. Maybe they would be ok with meeting with your therapist to get a better situation between you and them. It may not work and you may feel that you have no choice but to limit his involvement with them but just try all the alternatives first. Good luck to you and I really hope you can get somewhere with this!
-Jen