Supplements for soon-to-be 1 year old LO

Hello mummies. LO turning 1 soon. Would like to know when is best to start LO with supplements or if supplements are necessary. LO goes to Infant Care and has been falling sick every 2 weeks (fever, flu and cough). I am unsure if i should start introducing supplements to build his immune system, but i am unsure what supplements to give. Any advice/recommendations? Thank you.