Unusual Common Redstart behaviour

I found a Common Redstart nest last week. A female was tending the nest, keeping it tidy. A male was standing guard nearby. 

However, what was unusual was the fact there was an immature male, I assume from last year, also tending the nest. It wasn’t a one off, I first observed this immature male at 8:30am, and when I returned to the nest at 1:30pm it was still going in and out. 

Has anyone come across this behaviour in Redstarts, and any idea what is going on?

Thanks