My daughter got involved with a guy after having her first child. They married and she now has 2 more kids. However she found out he was having an affair and left him. My grandson is now 6. His “dad” takes his two kids to spend time with him but leaves my other grandson behind. When will be the best time to tell him that man is not his biological father?
Children need to know the truth, when it's happy, when it's sad, when, in this case, it's an ugly truth.
The man that the 6yo has lived with for years is his father, not by DNA but by the fact that this man was presented to the 6yo as his father, which has created a father-son bond in the 6yo.
The responsibility for this situation is with the man that was acting as the 6yo's father, and has left his son, and this is not something that can be changed or dressed up to be anything except exactly what it is. This is the truth that is the 6yo's reality.
The answer about timing is now. The longer the 6yo is left behind by the man he believes is his father, the more damage is done.
Start with a conversation about biological father vs the father you live with. For the 6yo, this is an important concept for him to understand as much as any 6yo can.
The 6yo needs to know that he's being rejected by a man that is not his biological father.
The 6yo needs to know why he wasn't told from the start, that this man was not his biological father.
The 6yo has a right to know the truth, but keep it simple, keep the conversation at a 6yo's level, and above all else reassure the 6yo that he is loved by adults that will never abandon him.