Pls how can I help my 3 yr old reinforce his immune systems, hes taking multivit but he gets sick sooo often. ..Thank you?

Please see below. The most important things are eating a balanced healthy diet, getting plenty of sleep, getting vaccinated, & good handwashing.Toddlers who aren't in daycare still get 6-12 viral infections yearly;if in daycare, that rate doubles to triples! if his doctor sees that he is having infections that are too many, too severe, or too unusual for his age, tests are indicated to check for underlying causes.
Allergies? Most common illnesses in toddlers are upper respiratory infections. Suboptimal allergy control increases chances of infections and cause similar symptoms. If 'getting sick' includes high fever, weight loss or other symptoms of infection in other systems, or occur > once/month, an immunological screening might be indicated. Vaccines are the only intervention that is proven to enhance immunity.