I read what you wrote about your son. I have to say, the message would be more coherent if you wrote in order the "symptoms" you observed and the steps you took, but never mind. RE: probiotics and antacids. People with frequent heartburn sometimes have actually not enough of a heliobacter pylori; however, too much heliobacter pylori can lead to ulcers. So... unless you are a doctor who has not only expertise but also the access to other practitioners for help, it is often best NOT to treat yiourself, let alone a child. By the way, mistreating a child medically can be legitimately considered a form of abuse.
Probiotics are bad when taken in excess. Also, different probiotics are useful for different types of problems, even if the problems are all within one area of the body, such as the intestines or stomach. It is, additionally, difficult to access information as to what different probiotics are good for specifically; holistic medicine has not reached such popularity that the information is readily available.
If you let me know where you are at now, I might be able to give you some guidance. I can try. In general, it would be unusual for a child to have yeast overgrowth in his or her intestines. Think about it: if it were all that common, you would see it on talk shows. Additionally, there are plenty of other microorganisms in the intestines; the intestines have more e.coli, just the number of that one bacterium alone, in them than there are stars in the galaxy. So if the flora of the intestine became unbalanced, there is no likelihood that it would even be candida overgrowth, because there are so many other more prevalent organisms to grow in the intestines. Additionally, Antibiotics , despite what you may read online, do not kill all microorganisms, they are not effective against all of them, let alone all of the ones in the intestines, to allow yeast overgrowth. There is also no likelihood that if there were fungal overgrowth that it would be candida, despite what people suggest.
One problem children do often have in their gut is pinworm; another is that they get constipated because they are too busy playing and having fun to toilet themselves properly. It can also be difficult to get them to intake adequate fiber (salads, grains) and sometimes fluid such as water.
However, if you keep self medicating your son without knowing what you are doing, I am sure he will develop something highly unusual and no normal doctor will suspect what it is because you will have introduced him to such unnatural conditions within his body that it will be hard to figure out what is wrong. So maybe it is time to cool it and let the doctors do their jobs. Before they think you are a Munchausen By proxy parent.