OP, it gets worse with the additional detail.
I don't necessarily expect the girls to get it - it takes a while before children develop empathy - but your DP is being awful.
Your DS had a mum and dad who loved him. His dad sadly passed away, and some bloke has taken his dad's place in his mum's heart, and in his home. Plus, he's brought along two young girls, who your son probably has naff all in common with. It's a lot of change that he didn't ask for and had to accept.
Now, at a time when he's missing his dad greatly and wants the love and support of his family, his blended family - the interlopers - don't want him there. That just intensifies the feeling of loss.
Young men can be quite vulnerable, and your son needs you. If DP can't accept your son coming home, he can move out and find somewhere for him and his own children to stay. Your son only has one parent left - you - and he needs to know you have so much love for him that he still has the love of two parents. You can't be his dad, you're his mum, but you can love him and stand up for him with the force of two parents.
Let him come home, have his own space, and be there for him whilst he works through this immediate crisis. If he needs to stay longer, well, that's a bridge you can cross when you get to it. For now, you just have to be there in his corner.
You know you're right to advocate for your son. That's why you posted. We're telling you your instincts are right, and you have to tell your DP to do one. He can accept that your son is coming home - to his home - and you can have a grown up conversation about what that means - or he can leave.