IMO the instant someone starts referring to “the first wives club” they lose all credibility. It just makes them sound bitter and really not very pleasant.
And let’s be honest here, only one poster mentioned that the child feels nothing for the OP and she was swiftly called on it.
OP at the end of the day while you’re personally in favour of this child spending more time at yours, and you’re prepared to be involved with his care (a lot of SM’s aren’t,) the reality is that your DH is using you as a means to gain more access when actually he’s not capable of parenting him himself.
If the DS wanted to spend more time with his dad then that would be fair enough, but being at yours more isn’t going to mean spending more time with his dad is it? It’s going to mean spending more time in a different house, and while you are totally prepared to be involved, he may not look at spending more time with his SM as opposed to his parents as something he wants.
And at the end of the day, most primary carers don’t gain that status on the basis there is someone there to help with the parenting. Even if they get together with someone else down the track, that is considered irrelevant to their position as primary carer.
Most primary carers don’t have a choice as to how often they parent their children, but your DP actively chose not to have him more than he does, which might have been fair enough when he was a baby, but he’s then used you as a justification for having him more. Not his wish to spend more time with him, not the fact that the court order hasn’t kept up with the changing age etc, but the fact he now has someone in his life to do the parenting for him when he’s not there.
And what kind of responsibility is that going to put on to you? I actually have no issue with step parents who want to play an active role in their SD’s lives, and I know that some people do. However, if your DS were to come and live with you on the basis that you will be doing much of the care, will you feel obliged to stay in the relationship should you ever want to leave, because you know that not staying will mean that your DH will again spend less time with his child?
These are all things to consider. It’s not just about the amount of contact changing,it’s about the whole dynamic changing