Because he's 14 months and has never been ill, not even a cold.
This man says for an immune system to work properly, it needs to be confronted with an infection in the first year of life.
This article says this, I've copied in a picture. This man is supposedly supported by lots of cancer research companies, and he has received or is going to receive an honour for his research and findings.
It's from The Guardian so less likely to be completely BS