My son, aged 2, has always been a tough squirt. He takes well to new situations and has no problem coping with mom now being dad and dad being mom. We still use our old terminology around others since we're not always passable (plus live in a conservative town), but he isn't bothered by it. If people ask "Why do you have two moms/dads?" Just say that, "We're just his parents and love him very much!" You don't have to go into detail or shame your son into thinking his family life is "weird". Just do your best to be the best mom you can be and keep open communication with your wife and son.
Just don't make it that big of a deal, but don't leave you family to deal with the hungry wolves. You have to find the balance as you start transitioning more. You might be surprised at how little your son is bothered by it especially if you've just not fussed over all the little things. Be supportive if he's upset by other kids but assure him that his family is normal just like the other kids' families. (Single parents, hetero parents, ect.) Before too long, not many people will get a second glance to gay/lesbian couples.
Most of all though, stay positive! All parents undergo drastic changes but they do their best to overcome. I wish you the best.