I agree with others saying that if you tell her, it should be because she deserves to know rather than to spite him (though I totally understand the urge!!)
I had a similar situation in that my ex started up with a woman, who knew he was married with kids (including a newborn) but I also knew he had spun the narrative, I was the psycho ex etc. In fact we were in marriage counseling and him moving out was supposedly so he could deal with his anger issues and mental health, sort himself out etc, then come home as a better person. I tried to contact her and she blocked me on everything. I had met her before as she was in his social circle.
I REALLY wanted her to know the truth and if I'm honest the reasons were two-fold. I wanted her to know because I would definitely want to know if I were her, but I also wanted to out him for his terrible behaviour and deceit.
The situation got more complicated as I fell pregnant again (so clearly, we were not actually over!) He said he couldn't cope with another baby; I said that's fine, you don't look after the ones you already have. I decided to keep the baby. Then I found out the pregnancy wasn't viable but I live abroad, abortions are hard to come by, I was told I had to wait til after amnio at 20 weeks to get an abortion even though the doctors all agreed with what the problem was... it was truly an awful time. I hid my pregnancy from my kids and colleagues, had the amnio, then 'luckily' miscarried before I had the abortion booked. I went to every hospital appointment alone, including for the D&C. He did pay for it. He claims he has told her the truth but I cannot believe that is true... it would take a special sort of fool to stay with a man if they knew all this, surely?!!
I still wish she knew but, to be totally honest, I have let go of trying to inform her because he pays reasonable maintenance and I don't want to jeopardize that. We wouldn't even be able to get CMS if he played silly buggers as we are overseas so he could easily leave me with nothing. So, I take the money each month, love my kids, and feel relief that I'm no longer with a deceitful, weak, selfish man, and that my children have very limited exposure to him. It doesn't matter that his friends and family don't know the truth and think he's hard done by and I'm dreadful. They are not important to me.
Wow... this went into an overshare 🤣 Feels good to write it down though!
OP - good luck whatever you decide, but if his maintenance is not official and he's not on the birth cert, maybe consider the risk of losing his financial contributions if you do take action.
And remember, YOU are the better person, raising your son and being a responsible adult. He is the selfish twat living a lie.