Your elder son has now become a reverse sibling. At our school DAO reverse siblings go to the top of the waiting list the day that the younger sibling is allocated a place. You need to contact Watford Boys and see if that is also the case at the school. I know several students who have joined the school as a reverse sibling when their younger sibling has gained a place. DG
This also happened with someone I know who's younger dd was accepted at SCD. The older was then also brought in to the school under the sibling rule. Definitely check in to it.