A male has to be 7 months before the AKC will register the litter, but it doesn’t sound like your neighbor cares about that. This seems like stealing to me, after all, many people charge stud services. I wonder how Judge Judy would try the case? If I were you, I'd report it to Animal Control, and I would get my dog fixed a.s.a.p., that one encounter may have changed him, and sex will not become much more tempting. So many unneutered dogs are killed by injuries due to car accidents; they can smell the female miles away. At 5 months, I don't know if he could have impregnated her, they can tie without any sperm, but no pregnancy results. I've read that puppies from a male that is too young often are deformed, not sure if this is true, I read it on YT.
I agree with the others, get him to a vet, he can get an SDT even if it’s the first time for both.