Definitely get a different type of antibiotic. The probiotics are good, but not from yogurt. Diary actually can cause intestinal issues and there aren't enough of the right kinds of bacteria in it.
What is he taking an antibiotic for? Depending on the issue and severity, he may not even really need it. For example, a pediatrician I work with learned that it takes 2 days for an antibiotic to take effect on an ear infection. And, get this, it takes about 2 days for an ear infection to clear up on it's own. So ask your doc lots of questions and see if you can't get answers that make better sense for your family.
Good luck...D.