Selfish you are good to have so much care for your step son. I think that is one of the less discussed parts of step parenting, the 'child that falls through the cracks'.
My eldest two DSDs were definitely this, their Mum, used to relying on DP for everything anyway, just stepped right back from most parenting when they separated.
I know it's not looked on kindly to moan about ExW, and I am an Ex too. But I've seen first hand what can happen when the Mother's selfishness and entitlement after divorce pulls all the resources away from the children, who are dumped with Dad who is still the provider and therefore just not around as much.
Then we as SMs come along, and instead of being rewarded or thanked for pointing out and noticing that these kids are NOT BEING PARENTED very much, if at all. Call is Disney Dad or whatever, it is the same thing, too much being wrapped up in their own fears and not even providing a stable, normal and dare I say, fairly disciplined environment!
My eldest two DSDs have both been the result of this, both sent to live with us but I was prevented from plugging that parenting gap. One got into a lot of trouble, all of which only I had noticed and warned both parents. Both parents and DSD made me out to be awful just because I raised concerns, and even though all of these came true, all of this was conveniently forgotten. Except now with DSD2 who is now going the same way...