I am not sure if this is what might be happening, but from experience, my DD went through something similar. When she was about 5 months, she started crying and screaming when my dad would hold her or come near her. Up until that point, she loved him. It was just a very strange phase she went through. After about 2 months, she was fine. And now at 2 and a half years old, you can't keep her away from her grandpa. They are best buddies. We never did figure out why she did that, but she outgrew it fairly quick. So possibly with your son, it's just some stage that he's going through for whatever reason. He probably has a reason for it, but obviously we can't read their minds, but boy if we could, lol. Hang in there, hopefully it will subside soon enough.