Hi. Give a healthy well balance diet Include nuts after 1 yr if not allergic. Give him his vita A dosage under immunization schedule. Omegas help in immunity.

Citrus fruits , green leafy vegetable,eggs , pulses , dry fruits

Contrary to common belief,NO foods,meds OR supplements can increase the immunity level beyond that determined by genetics. Immunity builds up gradually over a period of time by repeated minor infections. All children up-to the age of 8 years WILL fall sick 5-6 times a year,it's normal and part of growing up. Just give 3 solid meals a day including curd and Banana in the diet in addition to whatever you prepare at home and 3 milk feeds including one top milk feed using a spoon and katori and rest breastfeeding at this age.