Another thread has made me consider this.
DS 10 is aware of DH not being his biological dad. He hasn't seen his biological dad since he was a baby.
10 year old asked to call DH dad when he was 6, so DS (5) has grown up hearing that.
DS 10 changed his surname from my previous surname to DH and my current surname aged 7 so that isn't a way to bring the conversation up.
Should we tell DS (5) that DS (10) has a different biological dad, and if so how do we bring this up with DS (10) without it making him feel that it's significant that DH isn't biologically his dad but is DS (5)s?
Alternatively should we just wait until DS10 brings it up or chooses to tell DS (5)?
YABU - we should tell DS5
YANBU - we should wait for it to naturally be discussed