I can understand your pain…many of us here are undergoing it, if at a slightly different angle. The good thing is that you were never a JW and cannot be disfellowshipped.
Browsing through some of your previous posts, four years is allot of indoctrination for your son. Unfortunately, this video is about those who are just getting involved, a year or less, prior to the making a commitment.
Steven Hassan recommends appealing to the loved one’s authentic self, reminding him of happy times before the cult.
From a JW perspective, technically there is no reason for him not to have contact with you. After all, the Bible says, “honor your father and mother”. There are even magazine quotes that show a person should not have to choose between family and religion (ironically). Perhaps sharing these quotes from their own publications could stir him to stay in contact with you.
Note the scan. In particular, the subheading, "Balancing Family Loyalty".