C.S.
I'm so sorry for your pain and loneliness in this struggle. I'm a therapist, so here goes my best shot and giving you guidance:
You obviously know this, as you are already doing it, but your first priority is protecting your son -- with discrete honesty. When I say "discrete," I mean that you do not have to fully disclose your hatred for his father, nor your reasons for hating him. But you do owe it to your son to be honest about how your husband's meanness hurts your feelings. You can also tell him that when Daddy breaks a promise to him or says something mean to him, he can tell you about how that makes him feel and you will be there with him. You've got an opportunity here to teach your son that we cannot make other people behave differently (we would if we could, but we can't), but that we CAN tell them how they are hurting us and we CAN find a safe place with the other people in our life.
You cannot spare your son the pain of having a lousy father. But you CAN teach him to be resilient, to recognize his own inner resources and to rely appropriately on outer resources (like your love). And if you are able to teach him these things, he will be a stronger person for it later -- not a damaged person like your husband seems to be.
One other thing: If you don't have a physical reason for not knowing how to ride a bicycle (and other things that "only" his dad can do), learn. Better yet, have your son teach you -- or have a friend teach the two of you together. You'll be demonstrating courage and determination in letting your son see that you're working on things that you don't yet know how to do well. That's also a vulnerability that all boys really need to have modeled for them.
A book recommendation (for anyone with a son, nephew, godson, husband, ex-husband, etc.): Raising Cain: Protecting the Emotional Lives of Boys.
I don't know if there are any play therapists in your area, but if your son starts to manifest behavioral issues (seems angry or sad all the time, for example) in spite of all that you are doing right, play therapy would be a VERY effective way to help him sort things out on his own cognitive and emotional level.
Good luck!