You could get your sons immune system checked, if he has a weakened immune system then no probiotics, etc...
You will get all sorts of information on this website but please be wise because you have a tiny little person. The FDA warns probiotics can cause infections and, you should consider the risks before giving probiotics. If you look at youtube and type in Horizon probiotics, recent research from The Imperial College of London showed that ingesting probiotics can cause pathongenic bacteria in some people. I don't know about ferment able foods but again please be wise.