I'll be upfront. My son has Down Syndrome. He is four.
First, the positive thing is you were not being called a goat by a person who was ill informed, or just plain rude, about public breastfeeding. So that is positive. There was no discrimination there.
*If* the young man realized that you were indeed breastfeeding, he was simply making a comparison to what is a big part of his world--the goat farm. Or as the the pp said, he may have noticed how you were caring for your child just as the ewes care for the kids. This probably isn't the first time that he has made the realization that both human young and goat young are called "kids".
As a mom of a child with Down Syndrome, if my son made that comparison I would be thrilled that he put the two together.
But also as a mom of a child with Down Syndrome, I would not want my son to behave in ways that embarrass other people. Social skills is a life long learning process for many people who have intellectual disabilities. And, I strongly disagree with others who said the person is incapable of learning the difference. Perhaps they do not have much exposure to people with Down Syndrome, but I am sure the young man is capable of learning. In fact, his employment is most likely a constant learning experience for him.
Do I think you should call management? At this point, I don't know. I think too much time has past for the young man to recall what his did and learn what to do different. But perhaps not. If you do call management, be kind and realize the young man was not condemning breastfeeding, just observing something he thought was neat.
If it had been me, I would have acknowledged his idea in hopes he would pipe down.
Also, was there not anyone else working with him? That person is the one at fault for your embarrassment I would think. They should have had the where-with-all to say, "Hey buddy, keep your voice down please." Or maybe that person had no idea what the young man was talking about.
Anyway, it is a rambly opinion, I know. Sorry about that. I hope your experience does not discourage you from attending the farm again. I really do not think the young man meant to hurt your feelings. I think he was just excited--even if he did express it inappropriately.