What can I give to my 5 month old to build up his immune system?

Breast milk. No, seriously. It has a lot of antibodies which help to build his gut health and strengthen his immune system. I'm sure you don't intend to feed herbs to a baby below 6 months of age.. http://sg.theasianparent.com/how-to-strengthen-my-child%E2%80%99s-immunity/