Such communication should occur, if at all during the pendency of either case, between the respective lawyers representing each of you. This is especially because of the identification of a "conflict of interest". You don't, nor should you, indicate what you wish to communicate or why. Do not post any further details on this public website or elsewhere on the Internet. Speak privately with you own attorney about the matter and be guided by the lawyer's advice.
You should have your attorney initiate the communication and determine if your testimony will adversely affect you. If could be a problem for you if your testimony turned out to be untrue because that would be a crime. Since your attorney knows all the facts, he or she would be in the best position to advise you.
I agree with other counsel. Although you "can" speak to your son's lawyer but it is much better if you initiate contact through your lawyer. It is difficult to give any more advice because of the lack of details about the matter. Your lawyer is in a much better position to speak to your son's counsel because he or she should have much more information and knowledge about the case. Good luck.