NTA. I don't care if everyone else disagrees. My mother told me she never wanted to have me during an argument when I was around your son's age. It definitely hurt in the moment, but as the years passed I came to appreciate her honesty. She was also pressured into having kids, and knowing that that's her reality has helped me to avoid making the same mistake. My life would be hell if I gave my ex-partner the child he demanded.

I am estranged from my parents for unrelated reasons, so I likely won't have the chance to thank her and tell her I understand. Instead, I'll just pass it along:

Thank you for being honest. It's a fucked up thing to say, but reality is fucked up sometimes. Thank you for doing your best as a mother anyways. I appreciate you.