You will of course be worried, take a step back and look at this as if you did not know, and love, or not, as the case maybe any of the personalites involved in this situation. Five people , new country , new relationships parameters, everyone like a fish out of water, and competition for his Dads attentions. The girlfriend has an ally your ex ,the girlfriends son has an ally - stable relationship with his sister,whilst your son is on shifting sands in every sense of the word.
Try not to get sucked in as it will serve no purpose except possibly uniting everyone against you. I think your son is going to have to brazen it out or it will be a complete nightmare for all involved in the future. Speak to your son. Tell your ex partner you just want to reasure your Son that everything at home is the same and that you are really looking forward to him coming home and ask his Dad to maybe buy a cheap disposable camera for him ( this is a diversionary tactic ) so that can take a picture of the beach etc. the other kids so that he can show you all the lovely places he has been. Alert your partner first and ask him to put a paer and pencil by the phone so he can make a note of what you would like to see. Tell him you are a bit jealous that he is away ans so you would like to share in the memories by looking at the photos, making you HIS ally.
Hope it works, stay calm. Much love Sense.