He wouldn't go into care. Years ago DH had PR for youngest child (born in 2004) but not for oldest ( born in 2000, on birth certificate but not PR as rules were different).
His ex got very drunk one night and when went to drop off we had to leave with his DD as they were trying to smash car up.
She called police on us and said we had kidnaped her DD. We were sat in police station at the time. Police did come eventually to collect her from ours but this was 4am as her mom refused to allow her to stay but police advised they needed her to sober up first.
We went to court and were given EW (this was about 16 years ago). Slowly increase that time and now youngest lives with us FT and has very LC and her DD is NC.
They covered for her for years, minimalizing her drinking. They admitted since that at 13 they were having to pick her up off the floor after school etc.
They didn't tell us because mom had threatened them that they would have taken into care and she would lose them and they would never go to their dads because we wouldn't have them. She would kill herself if she lost her babies at it would be their fault etc.
We are so ashamed that we didn't know. FB was always full of the happy family. They were scared to tell us because they love their mom and wanted to protect her, even though they were with us regularly as well.
If you love you son and really want what is best for him then you need to step up now. You need to apply for mediation and the right to apply for court to obtain PR.
Without this, even if you call social services they may not engage with you because legally you are not obliged to know.
There is a process and you need to follow it for the sake of your son and his mom, if you genuinely want to help her. You can then also arrange proper contact through the courts.