Such a tricky one OP. I really feel for you.
I don’t have any direct experience with this, but I would say the sooner the better. Your about to embark on the teenage years. This will become more difficult the longer you leave it, but beware that he will use this against you both in the years ahead. You will need to make peace with that.
How long has it been since he saw his real father? Would he have any faint memories?
It depends on so many factors.
Think about how you as a child would have wanted this to be communicated.
If it were me, I would have wanted some communication from my real father to explain it to me. To understand why he had left, why he wasn’t in contact, why no one had told me sooner. I would be looking for reassurance that I was loved, and that the lie was not about my mother keeping things from me, that you were waiting until my father came forward in some small way.
This would be the opportunity if you can get his father onboard at all. A letter or an email even?
If that’s not an option, timing will be important. Surrounding him with family, not being told and having to head into school the next day etc. You need to consider how you can be physically nearby ready to answer his questions. Be there whilst he processes it.
If you’re not ready, can you get some support or counselling from a local service to help you through this? It’s something you will all go through, so it’s hard for everyone. This may also help figure out if now is the time.