How do i get help for my 10yr old son who is a pyromaniac?

I am the divorced mother of a 10 yr old boy with shared custody. My son has been diagnosed with AD HD and recently depression. My ex husband has not been informing me of all the problems that my son has been going through nor the trouble he has been getting into. I have my son every other week and normally he's a pretty well behaved boy for me. This past week when my son was at my house he had taken my other 3 children (ages 4,3,2) into my one car garage, gathered some dry grass clippings made a pile, poured gasoline from a 5 gallon jug (I keep for the lawnmower) and lit it on fire. My car was parked in the garage, the door was open, but the other children were trapped in the back of the garage with no way to escape. my son continued to pour gas from this jug onto the fire. the nozzle of the jug was melted from the heat. LUCKILY my neighbors daughter came to visit and saw the fire. They got it put out before any one got hurt or worse. At the time he was doing this I was downstairs in the basement cleaning and doing laundry. my neighbor came and informed me of what was going on. I punished my son. thanked the neighbors and had a talk with my son about fires and how dangerous they are. When my ex husbands wife came to pick up my son I did not mention anything about the fire to her. I know I should have but I wanted to talk to my husband about it first, and we wasn't due back from work for another 3 days. When he did return I explained what happened. He was pretty upset. We decided we needed to talk it over with his dad and figure out what to do with him. I let my husband do the talking cause it's not a good idea to have me and my ex discuss problems, a mediator is much needed. Once their conversation was over which I wasn't even a part of. I was informed that my son has done this type of thing before. I was informed that my ex now wants to lock him up for it. I haven't even had a chance to parent my son. I believe I can find him help through local resources without going to the extreme of having him incarcerated. So what do I do try to get into see the judge that handled my custody case and fight my ex in court. I know once my ex gets something in his head there's no getting it out. so I feel like I'm really stuck between a rock and a hard place. The worst part of it for me is that my husband is so upset at my son right now for endangering the lives of our other children he agrees with my ex. Could somebody please give me some good advise on this very sensitive subject? I need HELP!