Objectively, I think your partner has been thoughtless rather than deliberately malicious - and he has tried to mask his guilt by passing the responsibility on to you - a classic male trait, I know.
I think you need to have a calm talk with him, away from the kids. You need to be sure that this is just careless and thoughtless behaviour, rather than an indication of some deeper malaise in your relationship.
I have two step-daughters from my wife's first marriage, and we have a thir daughter between us, and I have always been extremely careful to treat them as entirely equal, you can do no other as a step-parent.
Have a talk with him, and then you will know how to proceed, and I think he should have a serious talk with your (and his!) son and make sure that the boy is clear about his position in the family.
Just because your partner did not mean any harm does not mean no harm has been done, and he needs to take an adult approach to repairing the damage caused.