for people who love the littlest dairy goats
No one can give you a definitive answer to this question. Genetics are a crap shoot. Every goat will throw kids across a spectrum with better and worse genetics. If it were a perfect science, then every champion would be throwing nothing but champions. Years ago I had a pair of sisters who were as opposite as they could possibly be. One produced almost twice as much milk as the other. One was the bossiest goat in the herd; the other was way down in the pecking order. I would say all the time that if they had not been born on this farm, I'd have a hard time believing they were even related, much less full sisters!
When it comes to bucks, I don't keep any buck as a buck if I have any questions or reservations about him or his mother. Only the best of the best get to keep their testicles because they are going to have so many more kids than any doe. A buck can sire hundreds of kids in his lifetime. In my herd this year, each buck sired five times more kids than any doe gave birth to. The quickest way to improve your herd -- or send it south -- is through the bucks you use.
It's hard to believe that any reputable or experienced breeder would have said that something structural in a goat is not genetic. Something "not genetic" would be a goat with short ears because the goat had frost bite that caused the tips to fall off. Or a goat has a crooked leg because it was broken and not set properly.
Based on what you've written here, it sounds like you know it is hereditary, and he can pass it along to his kids. So, the question isn't really about genetics. The real question is -- what kind of trade-offs are you willing to make? And that is just a question that you will have to figure out for yourself, based upon your goals.
To be blunt, why use him if you have doubts?
You will regret it if you do continue to use him. You already are having doubts about the goats he sired. You don't need to do this to yourself.
I agree with Deborah; the breeder was either ignorant or less than fully honest because the problem he has is clearly genetic in origin unless you know that his pelvis was broken as a young kid and healed wrongly.
There are alternatives for you. There are lots and lots of really good bucks "on ice" meaning that you can purchase some fabulous sires in the form of semen straws. Of course you would need someone to do the artificial insemination for you if you cannot do it, but I would suspect that that is not too difficult to arrange, depending on where you live. The advantages are that you do not need to maintain the buck, that you can get really fantastic proven genetics without having to purchase an expensive buck, and you avoid the possibility of your doe/does inadvertently becoming ill from a driveway breeding, especially if the herd of the buck is not tested.
A very good friend said to us that we should remember it is easy to breed bad traits IN to your herd, but it can be very, very difficult to breed those traits OUT of your herd.
Be brave, and though it is painful, put this boy out to pasture and find another sire. (My two cents worth....)
Additionally, it is difficult to tell from one photo, but it looks as though his rear end is not his only problem; it appears as though he toes out a bit, and his front pasterns look weak.
"Perfect at birth" is an interesting concept. One of my pet peeves are people who brag about how great newborns look. I refuse to do that because no one can look at a kid and guarantee anything about how it will look in the future. I have a finished champion that finished dead last every time she was shown when she was a kid. On the other hand, we've sold a doe without papers even though she had a junior champion leg. In fact, many of the most experienced breeders don't even show kids unless it's at a fair where they're going to get premium money.
I also gotta say that if anyone is seriously selling every buckling on their farm as a buck, they are not doing the breed any favors. But unfortunately, I know some people will do that because you can't sell a wether for that much.
One thing I didn't mention in my original response is that if I use a buck and I don't like anything about his offspring, he's history. Ten years ago I bought a buck and bred him to several does. Most of his kids looked like pygmies! :( I sold him as a yearling. I kept one of his daughters -- the only one that looked dairy -- and she turned into one of my favorite milkers. But I am not at all regretful about selling him. It would not have been worth it to keep using him when I didn't like 90% of his kids. And sometimes it's just a matter of how the genetics cross. I also sold another buck when he was three years old because I didn't like his daughters, but the woman who bought him absolutely loves him and the kids he's thrown on her farm.