I'm so sorry Mia--it's hard enough to deal with kid issues, much less unsupportive ex. The best thing you can do is do something to make yourself feel better (phone friend, get a Starbucks,?); then address B's issues (I only say this because I feel somewhat paralyzed when I'm feeling down).
I really don't think writing should hold your son back. First off, because he is young, his motor skills are not as advanced as his brain. Improvement WILL come with age and practice. Keep advocating for the skip (if you and B believe it's right). Work on writing this summer and keyboarding (Grinity had both a handwriting program suggestion and maybe a keyboarding one) in anticipation that he'll also physically mature/motor skills.
