My 3.5 yrs old son, often gets I'll cause of viral gastritis.

This happens to him 4-6 times in a year. Followed by high fever and vomitings and sometimes lose motions. Since morning he is suffering with the same.we observed it usally happens if he eats something out of routine like for example sabudana khichdi, samosa or something or shevpuri,noodles (but these items are often honemades) Or weather change.He is preterm baby. My question is Why it often happens to him?

What precautions should be taken to avoid this? Does he needs any supplements for immunity.