Talk to Gynecologist on Stopped Breastfeeding

Hello, my son is 6 months old. He stopped breastfeeding when he was 2 months old due to viral fever and flu. I tried everything but he didn't breastfeed. Then I had no other choice but had to shift him to top milk. Now I got to know that kids who don't breastfeed till 6 months of age will have low immunity. Is it true? What should I do about it? I started weaning also but this issue worries me so much.