Can your husband speak to the mom as well? She sounds like she set your mind at ease, I really think she or any other health care professional would be best to talk to your husband too. It's different to speak to a person than just reading info on the internet.
I would never suggest putting your child in harms way and I can completely understand the fear, but if this were my child I would not keep him from his friend. My uncle died of AIDS about 15 years ago (I wish so much that medicine had been as advanced back then as it is now) and I will never forget him or the impact he had on me growing up, and I after being around him I would never want to ostracize someone with HIV. The biggest reason for fear is ignorance (ignorance is not a put-down, it just means not having enough knowledge) and if your husband takes the time to educate himself on this he will probably feel a lot better.
Since the boy's mother is a nurse, I am sure that they take it very seriously and behave very responsibly. Your son can be taught to do the same, and it should not be a danger to him. At 8, they can learn to be careful. Good luck to your family and your son's friend.