How do I increase my baby's immune system? He is 6 months old and falls sick at least once a month.

Yes to breastmilk! Other than that, when my baby was small, I used to make sure she got plenty of sunlight, I used to also change her cot sheets ever so often and disinfect them. Always empty out diaper bins so that there isn't any bacteria lurking around. I even put her pillows out into the sun once a week. Also make sure the aircon is serviced regularly. These are little things we don't always think about.