Antisocial step son 😩

My step son is 5 years old and has been coming to stay at my house for 2 years now along with his dad and my son who is 5 also.

There has been 0 improvement in last 2 years, he does not want to be here, he distances himself, doesn’t want to get involved in activities, tried going on holiday with him too and he just isn’t interested. I’m at my wits end trying, he barely speaks to us half the time. No matter how hard we try 😩 it isn’t just me that notices this, my partner does too. He doesn’t get along with my son either. His mother is no help at all as she is really antisocial also and just shrugs it off when we mention it to her, to be honest very work driven and doesn’t really spend much time with him.

Any ideas on how to get him more involved with the family ? We have a baby on the way and he just isn’t interested at all, in any of it. It’s stressing me out, making such a strong effort and getting nothing in return.

His mother gives us no information on how he is at home so we don’t know if this is normal for him or just towards us ? Any advice would be appreciated.