DC came home from his dad's with nasty bruises on his back

[america]

I put him to bath and saw two nasty looking bruises on his back (about an inch wide) each, one of a really angry purple color.
Dc says that GF was lifting him to throw him on the sofa (as rough play) and that the marks are burns from their carpet. Apparently ex told gF off but still my child has bruises/burns on his back. what should I do? Ex doesn't answer his phone. I should add that I live in ex' homecountry and that DC will go to the swimming pool tomorrow and the bruises look bad.

Accidents do happen. Unless you think the GF hurt your child on purpose, or through serious negligence, I don't think you can really do anything.

Take photographs. Keep calling your Ex until he answers and gives you an explanation. Until that happens and you are satisfied with that explanation, your child is not to be in the GFs company.
It's positive that your ex had s go at her. If it was 'just' rough play then she'll be more careful in future.
But you deserve an explanation.

I would take DC to see a doctor in the morning. Both to make sure that your DC is ok and to have a witness to this in case (god forbid) it happens again.