Texas Attorney General child support review

Here's a little background - My ex and I divorced in 2006. In the original divorce decree we have joint custody (1 week with him and 1 week with me) so there was no child support granted.

He's a fireman so he's never been able to have them more than every 3rd day. Once I moved to another town, it was decided that he wouldn't get them during the school week because he didn't want to have to drive them to school the next morning (30 miles). So now he sees them 3 days a month. This also freed him to take a 2nd job as a paramedic in an ER. Now they are 15 and 13. My son is 5 months from getting his license and that's going to make my auto insurance go up a great deal. I finally started asking my ex 6 months ago to help me out financially.

After much begging, he started giving me $200 a month. I'm sorry but I can't feed two teenagers and provide for them on that amount. He is making $54,000/yr at the fire dept (no idea what he makes at hospital). He and new wife live in very nice home, 2 new cars, etc. Let's just say that we DON'T...we struggle to the point that I have to buy some of their clothes at second hand stores. Anyway, I went through the Attorney General and requested a modification. The mediation is set for 2/10. What can I expect and can he fight this since the original papers say joint custody?