First of all, your husband's dad might not even show up for Christmas. He hasn't done so thus far and you've only spent 30 minutes with him which was 4 years ago. I guess I'm not sure why leaving him alone with your child would even be a consideration. He is a stranger to you and to your baby.
Do you need him to babysit or something? I aplogize....I have a bad cold so perhaps I'm not understanding what you mean by leaving the baby alone with him.
As far as cutting off all contact with your father, I think that is carrying things way too far.
I would get information from the pediatrician about AIDS and safety precautions to take. At a quiet time with your dad, just the two of you, you could maybe share the information just to make sure that you follow things as precautions.
I'm sure he would not want to cause any harm to your baby and as long as you don't intend to leave him alone with the baby, there should be absolutely no reason to end all contact.
I know someone who contracted AIDS from a blood transfusion. He came from a HUGE close knit family. If he had been cut off by his family out of fear, that would have been an even bigger death sentence for him.
Fortunately for him, his family embraced him as opposed to pushing him away.
It seems like a simple thing to agree on. You agree not to leave your baby alone with strangers and you agree to supervise interactions with your baby around others.
Best wishes.