Is this Custodial Interference?

Long story... sorry (aren't they all?)
Divorced last November, joint custody one week per parent. My son (15 yrs. old) was homeschooled at the time. Had been homeschooled since Kindergarten. After Dad moved out, he enrolled son in public school 2006/2007 school year. Did not check with me at all. Son stayed with Dad Mon-Fri. since he was enrolled across town and I could not possibly provide transportation with my work schedule (Dad made his own schedule). No divorce/custody agreement yet, I went along. In December, I found out my son hadn't been to school for 10 days in a row. He wasn't prepared for public school environment, didn't like it, so he wouldn't get up and go, and Dad didn't make him. We went to mediation, came to no agreement. The following year in July my attorney removed himself from my case he said for non-payment. (I hadn't heard from him or seen any progress on my case for 6 months, so I stopped making payments). In November I agreed to allow Dad's attorney to draw up papers to cut debt and custody down the middle with no child support just to get it over with. I checked into a school, Dad said go ahead - "if son agrees". (you see the pattern here, son gets what he wants with Dad) Found one, (in February 2008) tuition is $535 per month. In February I started keeping son more than just my weeks because Dad requested it since he didn't have a job and could not afford to feed, gas to and from school, etc. (Dad is awaiting a "settlement" from lawsuit) I have paid over $500 this year in medical bills. For awhile everytime I got my son from his dad, he was sick. There is no medical insurance, I work part-time. Divorce agreement states we pay 50/50 on medical, claim son alternate years on taxes. I told Dad his half of tuition. Still haven't received a dime for tuition or medical, and I think Dad claimed son 2 yrs. in a row. Now Son and Dad requested he stays with Dad Mon.-Fri. until school's out since he has to go to school earlier when he's with me because I work. I agreed (since there was only 2 weeks left of school). I found out yesterday my son didn't go to school (only one day so far as I know). He said this was because his dad needed his help on their new rental house. At his dad's he has no "bedtime", he does mostly as he pleases, and is following in Dad's footsteps. Examples: stays up until 11 or 12 at night, bathes and brushes teeth maybe 1 or 2 times per week, surfs **** on the internet, etc. Again, he is 15 years old!
QUESTION: At this point do I have to hire an attorney and go back to court? If not, what is my next step? His dad is an ex-police officer (has been "blacklisted" in our area and can't get hired on any police dept. locally) He has a history of intimidation and throwing his weight around. He makes his $ nowdays through odd jobs so his income can't be tracked. I am only trying to give my son the chance to get on the "straight and narrow" before he turns 18 and goes out into the world. I am afraid if he doesn't straighten up, he will be in jail by the time he is 18. His school has been a WONDERFUL influence on him, but how do I make sure he actually goes when he's not with me? Naturally....if asked....he prefers to spend as much time at Dad's as he can.