When is your DH back and what are the plans for him seeing his DS? What contact have do they have while DH is away (whether that is holiday or work?) Do they facetime daily, play online games etc?
It sounds like DS had to choose between travelling with the two of you, or leaving his mum over Christmas. That must have been really hard for him. That's not saying your decision to go was wrong, far from it. You were travelling to see elderly relatives, and presumably a lot of the "holiday" was assisting with care etc and helping out more local family. However your DSS must have felt really torn, and as we've all mentioned, the hormones will be playing a part, probably a big part.
Despite the fact that you were nothing to do with the break-up, he may secretly believe that if you had not arrived on the scene, then his parents may have got back together. However incorrect this belief is, it is a common belief amongst children whose parents are not together.
It's lovely of you to offer to help, but he really, really needs his Dad at this point, by the sound of it, and your DH needs to support his ex, in her role as their son's mother. You describe her lifestyle as chaotic, so I presume she has quite a lot going on at present.
I also wonder if DSS would benefit from some sort of counselling/therapy at this point, to unpick his feelings about all of this?