[Solved] DNA Test - Suspicion on Parentage

I had a child when I was younger with my ex (I was 20 she was 21). The circumstances of her conception always seemed a little bit suspect to me - I would go out and work all day and she would stay home with her older child from a different man. Whenever I asked what she'd been up to she never really answered me - I had no idea what she did all day to be honest. We were only together less than a year when she very suddenly pressured me heavily to have unprotected [censored] with her on one occasion and 2 weeks later excitedly told me she was pregnant, she was keeping it and don't even ask her to consider an abortion. I had never wanted children, especially not so young but we agreed I'd be a part of our child's life.

Fast forward, several years of manipulative behaviour, obstruction, a court case and CAO later - I no longer see my son even though it was court ordered I should, she and her overly involved parents alienated him from me so heavily that I had to forfeit all access just for his mental wellbeing (he started to become very distressed). They have always been adamant they don't want him to have a normal 'father and son' relationship with me, refused to refer to me as his dad and would almost shudder if I ever asked why they would be so cruel to my child by confusing him about his parentage, they told me several times 'well you're not his father, not to us' and it never seemed normal. It became evident when she kept bringing her dad along to collections and pick ups that she had also fed her family a lot of lies about me to explain to them why she didn't want us to see each other. I wonder if all of this has been a front to cover up her cheating on me, or having a child with someone who perhaps wouldn't pay up (she had very undesirable friends when she fell pregnant).

I have today received his school report and it has a photograph of him on the front - it is shocking, this child looks absolutely nothing like me - the more he grows the even less I can feign any kind of similarity, he looks like somebody else's child. It could just be her genes were more dominant, but thinking sensibly he has never looked like me. I am suspicious he is not mine. I had tried to do a private DNA test during my interim contact but the lab never got our samples and so I never got a result - by the time I found this out it was too late as I no longer see him, I'd have preferred to do one privately and her not know, if it had come back with any adverse results I'd have then pressed for a legally binding DNA test.

We pay maintenance via a family based arrangement so currently CMS are not involved, I send her an agreed amount once a week and we've always been happy with this arrangement. She never asked me for money, I insisted I pay for my child. How do I go about requesting a DNA test from her? She wouldn't agree to one on friendly terms - she is always extremely hysterical and obstructive so I'd like to do it in a formal way where she can't put her parts on. I'd like the peace of mind he is definitely mine - especially after all she has put me and our son through.