Your situation sounds a bit complicated and there are a number of important facts that aren't in your description. As a general matter, if you and your husband are separated he probably cannot claim you as a dependent on his taxes; he may be able to claim your son, but there are a number of additional facts we'd need to know before anyone here could give you a good answer.
As to your second question: assuming that your husband could claim you and your son on his tax return, there is not much you could do to force him to give you half of his refund unless the separation agreement says that he must give you half of it.
Given how fact-specific this sort of question is, you should really consult with a competent local tax professional, such as an accountant, in order to get an answer you can rely on.
My answer does not constitute legal advice and may not be relied upon by anyone for any purpose and does not constitute an attorney/client relationship or an offer to form such a relationship. This disclaimer is intended to be fully compliant with the requirements of Treasury Department Circular 230 and the terms thereof are fully incorporated by reference. If you wish to consult with me please contact me at dana@nytaxcounsel or visit my website at www.nytaxcounsel.comAsk a similar question