Filed c/s modification, now he wants full custody. Advice please

[formykiddos]

I have filed a motion for c/s modification due to change in circumstances. Our daughter was diagnosed with diabetes type 1 in October. Medical bills, food bills, transportation bills etc. have increased. I have been unemployed since April 09 due to business closure. My fiance' supports the household and I receive minimal unemployment and court ordered c/s of only $400.00 a month for our 2 children ($200.00 every 2weeks), we agreed on this amount 3 years ago because I didn't want him living in squalor when the kids visited every other weekend.  He also pays child support for one of his other children (5 kids total..2 are ours, he never supported the others). I knew his take home would be minimal. My reasoning at the time was that I didn't want the kids to be with him in an unsuitable environment.
Since the divorce he has been unable to provide a stable place for the children to visit and he has been in and out of homes and relationships. He has a good job for the county, he makes quite a bit more than he did at the time of the divorce and I think now he can afford to help the kids more and still maintain his lifestyle. I have primary physical custody and we have joint legal custody. The kids see their dad every other weekend for 1 night it should be 2 but he works graveyard and cant be home with them Friday night, so they only stay Saturday night. He occasionally takes and extra day off so they can stay 2 nights but not often. He does not have them for the week of Christmas or Easter vacation like he should because of his graveyard schedule (I'd rather they were home with me anyway).
He does not reimburse me for co-pays, I do submit receipts to him (divorce states we share those expenses) he does not buy anything extra for them like school clothes,new summer clothes etc. I have asked for more help several times before and now that our daughter is needing more, but he has refused. Saying he is broke, Yet he has two rock crawler jeeps and goes out of town often to support his 4X4 lifestyle and he updates his web sites that show his jeeps and all the NEW parts he has added and trips he has taken. I don't see how he is broke but whatever.
I filed a motion for child support modification last week. He received the papers this morning and called (I let it go to voice mail) to say angrily he is filing for full custody and I am an unfit mother being unemployed and not providing for our children, he then continued to say I am trying to EXTORT more money from him. I cant believe he thinks paying child support is extortion. I have tried to maintain a tolerable relationship with him even though our marriage was verbally abusive. I am letting calls go to voice mail because I know he will threaten me in some way. I had a restraining order on him for death threats 3 years ago before the divorce and will not hesitate to do it again, but I do not want it seen as retaliation by the judge. I do however fear him.
He doesn't want to pay more support for his children and he is going to lie and threaten me to get me to back down. I am concerned about the kids and what he is going to say to them...our daughter knows whats going on (she is old enough and understands its for their benefit) I think he will try to get our son to go and live with him. I am representing myself in this motion, I cannot afford an attorney at all. I feel if he is able to afford an attorney to try for custody, It again proves that he is able to afford more support. In the beginning it was not about support, I wanted my kids! I was a single mom making great money and I didn't think I needed that much from him, I was more concerned about where they would be living when they were with him. So please don't think its about the money, if it was I would have fought for full support from the beginning. Circumstances have changed and the kids need the help now. Plain and simple.
I am a good mom the kids have been with me and raised by me and they are happy my son is an honor roll student and my daughter is dealing with being newly diagnosed diabetic. My fiance loves them and treats them like his own. They are healthy happy children. (no abuse neglect drugs drama etc.)
I have tried to cover all the bases with this post and answer all the questions that may arise. I do understand that this is his Reaction to my Action and he is blowing off steam and making his usual threats. However the threat of him taking my kids is very real and unsettling to me.
Someone PLEASE re assure me that I don't have anything to worry about.
Thank you to anyone who has advice or words of wisdom.