This very much reads to me as though you were happy with the status quo until your child arrived OP and now suddenly expect things to change.
Which really isn't fair, or reasonable.
If he suddenly changed what happens with his eldest, that could and probably would cause resentment, and could permanently damage their relationship.
I don't think 24 hours in a week is excessive, and I don't think that the fact your son has his dad most of the week in the evenings and presumably when he has some annual leave means that he's missing out.
It's not an easy situation, with there being barriers and divides involved and I completely appreciate that. DP goes and sees one of his DDs at her house and takes her out as she doesn't like to stay away from her Mum so we do things that way instead. It went on every weekend for a few years until she got to the teenage years and now lets him know if she's free or not.
So much as it's tough for you, I'm afraid that it's just how it is. Your son is important, but I'm not seeing where he's being discriminated against (fucking ridiculous statement btw, whoever posted it has clearly never experienced actual discrimination) nor is he being sidelined.
It all reads as though you're not happy that he's spending time with his other children and feel that if they won't come to your house they should be excluded.
And as for feeling like a single mother because for one night a week your DP is parenting his other child, I think that is ridiculous and overdramatic.
He had kids before you did, he should absolutely be as involved with them as he was before you and he had a child.
From what I'm reading, you're the only one who has the problem with that.