Building immune system

My toddler has been going to daycare since January (15 months old) and in his short time there he’s gotten sick with something every month. I was a dummy and didn’t start vitamins prior to his start (during flu season smh)

Even when he’s not sick he sounds sick... Any advice on ways I can battle against daycare germs? Vitamins? Probiotics?

I bought these multi vitamins but lately he’s not interested 🙄 also little critters gummy vitamin if I’m lucky he’ll eat one.