Telling DS5 about his birth father

[roratone]

I left my eldest child's father as a tiny baby. Moved 100s of miles away to be nearer my family. No contact from eldests father ever. When eldest was 18m, I met my partner whom I have never lived with. He is called daddy by eldest. I have another child by him who is almost 2. This relationship has pretty much faded out. He sees kids twice a week - lives 45 mins away. I'm thinking if we do split, he might only see the little one. Nothing to back up this suspicion at all by the way. But it got me wondering when and how i should tell the eldest he is not her biological father. Youngests father has a very demanding job which I feel is prioritised which is main reason for the split.