Hello from JustAnswer.
The usual next step in assessing for psychological abuse would be an assessment of the granddaughter. Since your son does not have custody, it may not be as simple as just taking the granddaughter to a psychologist. It may be necessary to petition the court to ask for the assessment to be done. However, at this point, it is as much a legal issue as a medical issue, so asking his attorney that has been representing him through the divorce may guide how best to get this done.
I I can provide any additional information, please let me know.