Hello. I can't answer medical questions. This forum is for people to discuss emotional or psychological concerns.
If your husband's physician has prescribed Viagra for him, then she/he must have determined it's safe for him. It's very dangerous to take Viagra without a prescription because it does have side effects which can be dangerous.
I can't answer your questions about having children because there are many variables. Generally speaking, if your husband is producing semen with viable sperm, and you're producing eggs, your chances for pregnancy are increased.