Whatever reason your husband has for not having spoken to him is not a good enough one to shirk his duty as a father. I agree with you that this would be better addressed by Dad.
I don’t understand why you don’t know how much your husband has said to your son. Talk to him, have an honest discussion. Ask him, “How much have you told Junior?” Then when you have a handle on that, if you think he needs to say more, get into more detail, tell him that. You two should discuss it, agree on what needs to be discussed, and in what way it will be discussed. Then tell your husband that you have complete faith in his abilities as a good father and that he will fulfill his obligation to educate his son. And this is not a one time discussion. Your child needs to know that he can approach you with questions and get good, solid, information in a calm and loving way. Not red-faced embarrassed stammering that leaves him feeling ashamed or awkward. Because that leads to getting stupid information from friends or searching the internet… and I don’t think much good will come of that!
I guess I am a bit sensitive about this topic, because my husband’s parents did a terrible job (more like non-job) of discussing sex with him, and he paid a terrible price in bad adolescent decision-making and living with the consequences of it for years. He hated himself, was depressed, wrote suicidal stories… I am still angry about that and I urge other parents not to leave their young men unsupported. I think in some ways they are even more sensitive and needful than young women.