Ok.. coming back to you from a step parent POV.
No, they aren't uncommon.
No, i did not love my step-daughter like i love my biological children.
Anyone who says otherwise is a better person than me, or a liar, quite frankly.
However. Put that aside for a moment, and come back to why i said he needs to sort himself out.
It is HIS responsibility to work through those feelings, without taking it out on you or your son. You, not your son are responsible for his feelings, nor fixing them.
This comes down to what kind of man he is.
I worked hard at first to treat my DSD like the kids i've had in my care (i'm a TA/nursery nurse) to treat her like humans who need my attention and care, to respect the fact my partner and then husband loved her dearly.. a love i understood once i had my own children with him.
I never once complained she was an inconvenience when she came to stay with us, i actively made sure i spent time with her building a bond, without trying to step on her parents toes, i included her in our lives and our holidays like she was meant to be there - which she was. I accepted that even though i now had my own babies, that she was as important to my DH as they were to me, and made sure to include her, and show her love.
To that end.. i have since divorced her dad, she is now an adult. She still speaks to me, to her Mum, and her Step Dad.. she has gone NC with her dad/My ExH.
Your DP/H (Whichever he is) needs to stop listening to everyone stroking his poor me attitude of how haaaaaard it is to love a step child and how normal it is to feel like he does, step up, and stop being a cunt.
If he can't do that, then he is a shitty person, and you, and both your sons deserve better.