Chrissy, you need to move. If only for your own piece of mind and to establish your own parenting skills.
You live with your parents, you are their child. Your child is their grandchild. Obviously ;) but they feel like it is all their responsibility. Your development and, ultimately, your son's development. Even though you are his mother, your parents are in the mix too. Feeling just as responsible as grandparents as they do being your parent. (i'm not saying what they are doing is right, i'm merely giving ideas as to why they are doing what they do.)
if you go away for the weekend and leave your son with them, what do you expect? I had loads of fun at my grandma's house. She let me have as much ice cream as I wanted and I got to stay up and get my way every time! But I didn't live with them. That was only a treat or a special occassion kind of thing. Not an every day all day occurance.
If you don't want your son to have pop, then say no. Mean no and make it no every single time. Not just for right now. But for every time. My nephew is clinically obese at age 4 because his own mother cannot say no. She says he gets into the fridge on his own and her parents always give him pop whenever he wants it. As the parent it is your responsibility to make the rules and stick to them. Just as diamondz said. You are the parent. You be the parent.
Of course everyone will rebel at first. From your son having tantrums to your parents having their own tantrums. But if you want things to change, you be strong and change them.
Good luck ;) (i know all about this. I lived with my inlaws when I had my baby at 16. It was not a pretty picture!)