Hi - my name is XXXXX XXXXX I'm a Family Law litigation attorney. Thanks for your question.
These are always difficult situations, and it is very unfortunate that the children get dragged into the middle of the legal process. However, that's the reality of these kinds of cases.
In your situation, yes, you can testify about what your son told you because you have first hand knowledge of what your son has told you. But, you can't testify about what his mother said or told him because you don't have any first hand knowledge of that - - it's hearsay. Your son, on the other hand, can testify in court and tell the judge what his mother has told him. Usually, the judge will allow the child to testify in chambers (outside of the public courtroom) to make things as easy on the child as possible.
As for the attorney questioning the child, he/she is probably trying to find out what the child really knows, how he really feels and to decide for himself if he thinks the mom has coached your son. The attorney knows that the judge is going to ask your son if anyone has told him what to say, promised him anything if he says what they want, etc., and her attorney doesn't want to be taken off guard and have the "deer in the headlights" look.
You may want to ask the judge to appoint a guardian ad litem to represent the son and his best interest to make sure that he isn't influenced or taken
I hope this answers your questions, but if you have others, please REPLY and I'll be happy to respond. Thanks!