Wow that must have been really really tough for you being abandoned by your mother when you were only 14.
Have you ever had any professional support to explore, heal, resolve this huge emotional wound? I would also suspect that your life before that with you DM was not 100% emotionally nurturing.
You sound like an incredible Mum to all 4 DCs but have been let down by a series of important RS in your life. That’s really hard for you.
Finishing the RS must be hugely triggering for you as DSS is more or less the same age as you were and you can also see how vulnerable he is (and how you were at 14) - but know that whatever decisions you take or arrangements you come to with his father you should not feel any guilt because you have not and will not be abandoning this child.....you have done so much for him emotionally and educationally that no one can take away from him and will last him for life.
If you choose that his DF has to move on and out and his DS goes with him - this is not abandonment - as you can maintain a RS if he isn’t living in the home - it’s what happens when any parents separate. Also at his age (GCSE year) other young people move into digs for apprenticeships, go to residential colleges etc - from 14 - 18 I stayed at my aunt’s quiet house M-F to be able to study away from the chaos of my family situation.
I would lose the guilt - it’s just crippling and distorts your thinking - and would think what works for you and your DCs first in the short and medium term and then decide how much or how little you want to offer to DSS. Be confident and clear headed about this. DSS will have his own ideas as will STBXP and the dynamics may change over time.
But you have every right to move STBXP on ASAP.
This is v emotional for you - don’t let the guilt simmer into contempt and resentment. Work out what you and your DCs need first and foremost.