Well good timing I've had the counselor yesterday.
DEX is all for me having DS1 for christmas and for a week of the school holidays. But if he doesn't want to he doesn't have to apparently. Nobody will enforce a teen to visit their estranged parent as there is a huge runaway risk.

I don't believe for a second DEX is actually being supportive and encouraging him. I don't believe for a second DS1 would run away either (Too lazy to fend for himself)
SO it seems I'm stuck between a rock and a hard place.
I've also dug in my heels and made sure if he does come there will be someone else here to back me up that I'm not cruel and abusive towards him. DS1 is not happy about that and claims he won't come to my place if there is someone else there. So we will see if DEX actually wants to be supportive and encouraging as he seems to have other plans.