Can he take my son away from me?

My soon to be ex-husband emotionally abused me for the last 4 1/2 years, the entirety of our marriage. I finally found the courage to leave and divorce him. We talked about a week ago about custody and et distribution and we both agreed that I would get my 3 year old son for 10 months out of the year, and he would get him for 2 months every summer as long as he is not deployed. We also agreed that we would alternate holidays with our son (ie 2011 my ex gets him for Christmas, 2012 I get him for Christmas... and so on with each holiday). I just talked to him a few minutes ago and he told me that if I have to go on any kind of public istance program (ie. welfare, food stamps, rent istance, wic etc.) that he would take my son away from me and the custody arrangement would be reversed meaning I get him for 2 months in the summer and he gets him the other 10 months. Is this legal? Can he do this? Is me going on public istance saying that I am an unfit mother? Please help!!!