whats done is done, not point calling out the OP and her DH now.. the thing is what to do now?
as many have already said, it might be best to stop forcing the title Step Dad onto your son, allow him the respect to call DH by his name.
you have most certainly made it clear who was more important, and it wasn't your son, but what to do now i have no idea.
maybe you and DH need to make sure that the son knows that DH is not there to replace his dad, and that even just 'friends' could be enough.
my own DD's refused to call their Step-mum anything but her name, EXDH and SM were more than happy with that, my DD's never ever call my DH stepdad, he will never be their dad, and they call him by his name and accept he is my husband.
perhaps the OP needs to spend time with their son without DH present, maybe even consider some for of counselling for both of you. You might be able to turn this around, but 14 is a tricky age!