Well, my Dad wasn't gay, but he did move out when I was in late middle school, so I have a notion of part of what your son will be dealing with. A few general points:
1) Divorces can actually be handled well, but a lot depends on the attitude of the adults involved. The week my Dad moved out was a tough one. Then I realized that my life was actually better with my parents no longer living in the same house. I saw more of my Dad after he left (1-2 evenings a week, Saturday afternoons, and he even came over for some holidays) than in the 6 months or so before he did. My house was no longer filled with tension and parents lying to me and saying everything was fine when everything was clearly not fine. But my parents continued to get along well enough to sit next to each other at school functions and not snipe at each other. Not all parents can manage that.
2) If your son's classmates are going to tease him for having a gay dad, they'll do so whether or not you're physically close by. If anything, moving further away seems likely to make it a bigger scandal as people are left to imagine the sort of life you're now leading, rather than seeing you go through the normal everyday reality.
3) Make sure your son knows that while there are limits on how he acts, whatever internal emotional reaction he has is acceptable. Maybe he'll be mad at your boyfriend. Maybe he'll wish you could have just remained in the closet, or been straight all along. Maybe he'll be hurt, maybe he'll be fine with it. Maybe he'll be scared about his family being different. Maybe he'll be sad that things aren't how they used to be. Maybe he'll be proud that his Dad was willing to face the truth even if it cost him a lot to do so. Maybe none of the above will apply. Any of these are fine. Just don't try to tell him how he should feel, and do let him know that you love him even if he's mad at you.
4) As far as your socially conscious mother goes...what really matters is what's best for you and for your son, and I'd even say your ex wife comes higher in the priority list than your mother. Your divorce and orientation have nothing to do with her.
The fact that you're trying to figure out what to do is a good sign. But you're the one who has to make the decisions here. Bottom line, weighs the likely pros and cons, pick what course of action you think is most beneficial to you and your son, accept the possibility that you might have picked wrong but you did the best you could, and just move forward from there.