My 5 year old German shepherd husky has a problem with something touching his butt or back legs

I have a 5 year old German shepherd husky that doesnt like a foot or leg touching his butt growls and barks but not all the time he is fine if he lays down and someone's leg or foot is by his butt, he is fine with that he also loves butt scratches my main concern is we have a baby coming next month and my dog growled at one of our friends babies because the baby touched his butt it doesnt seem like a pain thing but more of a bad touch spot I was just wondering if there is some sort of training or something we can do to correct this behavior or if it needs a vet visit for possible hip issues