If my son's ex says he's the father of her son, but can't prove it. Can my son get a court to order the test, as he's been trying and she won't do one .

Is the ex asking him for support or anything? Or, if the child is his, does he want to be a part of the child's life?

If all he is looking for is to know whether the child is his or not, then no, the courts may not order a test. But if she files for support or if he wants to file for joint custody, then yes, he can get the court to order a test.

Assuming the child is still a minor ( you did not say ages) and if your son wants to file for visitation with the child, he can as part of that court action ask for a DNA test to prove he is the father.