OP I agree with you, his attitude is not healthy, and unless he changes his behaviour, it will not be a healthy environment for your little boy to be brought up in.
For what it's worth I think most of the threads by and about step mothers are also not healthy for the children. Marry a man with kids? Be an actual step mum. But to be fair most of these threads are because the Dad is being a useless Disney Dad and expecting the woman to do all the childcare, all the drudge work, give up her holidays from work to mind his kids etc.
I would ask him how he sees you all being a family. How he thinks the two kids should relate to each other. How the two of them learn about security and family love. The effect of jealousy. Talk to him about your need to ensure that BOTH your children feel equally loved, secure and wanted within the family.
I used to work for a homeless charity. The majority of teen boys on the streets and in hostels were there because step fathers had made them unwelcome and as soon as they hit adolescence / approaching adulthood, saw them as an actual threat.
I don't care what anyone else accepts for their family: in mine I would see my step kids as MY step kids and as children of the family, equally my responsibility. And I would not accept a man who did not take the same approach with my kids.
Your DP has 4 months to sort himself out.