This is a really hard dilemma, basically, you have to choose between your boyfriend and your son. It sucks.

I personally, don't think children should be raised by their grandparents, especially if she's 80. I think that you, your son and your son's father should talk this through together. If the father of your son is willing to let him move in for 3 years and have the time and means to take care of him, that might be one good course of action.

Another good option would be to move your kid, even though he might not want to at first, with you to Florida, it won't be easy, but it might be the best solution all around. I think that living with the mother he's used to and known his entire life is maybe more important in the long run then staying with the same friends and school.

I think that in both cases the first thing you should consider is how serious your new boyfriend is about you and what are the chances of that relationship is to succeed. You are breaking your son's family apart my moving away, whether you take him with you or not, and unless your new boyfriend is 100% committed, as  you are to the welfare of your son, then maybe the best thing to do would be to wait till your son's old enough to take care of himself and then think about new marriage and moving away.

I don't think you should give up on your chance of happiness for you child's stability, but I do think you need to be sure that the new relationship is sound enough to do something that will effect the lives of your son and his dad.