How can I?

How can I talk to my (almost) 10 year son about why I sent him to live with his father at the age of 3.5 b/c I severely depressed and on the brink of suicide? His father initially didn't want to take him but once he realized he wouldn't have to pay child support doing that he called and said he'd take him.

I got myself together and after about a year of him being with his father...I said I was ready for him to come back. I had proven I had gotten help (which was diagnosed as late term Postpartum depression)He said okay and then ran with him when I made it to his state! I petitioned the court and despite the circumstance they said he had already been there for a year so I couldn't do much but I petitioned again! In total we were in court for 2.5 years making 3.5 years my son had been there.

We finally meditated and I agreed to allow my son to stay because I found out I was pregnant with my 3rd but making 12.50 an hour. Unfortunately, he got to use his and his wife's income so it looked like I wasn't as able as he was! To be very clear I was in no way found to be unfit. They kept dragging out court in an attempt to try and find dirt on me (found none) and even went as far as lying and saying a family member of mine molested my son. I was notified 6-7 months after this alleged event, keep in mind we live 14 hours apart. Thank our divine and protecting God that an officer assigned to the case studied it and said it sounds made up. He then had his chief review it and they were both the professionals and go to for such cases and when given the background story the officer said he has seen this over and over and over again! He dismissed the case and closed it.

Anyway, I've since healed from all that! This was almost 3 years ago so almost 6 years my son has been there

My issue is whenever my son comes to visit he is telling me he wants to come back and live with me but his dad and stepmother have told him no because it's not safe for him and that the judge said he couldn't! I wanted to call and curse both of them out but they don't even matter. I felt like a POS mom but I sat my son down and in the most simple way told him the truth. That mommy was very sad and needed time to get better and if the judge truly felt he wouldn't be safe then I wouldn't be allowed to see him. I also told him that I love him and I always will no matter what anyone says about me to him to know I am his mother and I love him!

Idk how to defend myself against them bashing me to my son. My son frequently says he loves who I am despite what I've gone through but he shouldn't know that! I've spoken with a family attorney and was told it's too difficult to prove parental alienation..because that's what it is.

So again, how do I talk to my son about this? How do I defend myself against the hate?