Donor sperm baby, when/how to tell

[Nofunkingworriesmate]

Son is now nearly 5 and was made using an open donor from abroad.
He has two mums but has started school so is now more aware that all his friends have daddies and has expressed desire for a daddy😕
We were going to wait for questions and deal with them as they came but we are now concerned that that's a bad plan. We have ordered books off amazon as a starting point, has anyone met/ plan to meet their donor successfully?
Any advice gratefully received ...

My son is 6 and conceived by donor. I've always talked about it in an age appropriate way with him, so up to this point he's never really questioned his conception. I didn't want it to be something I had to do a big reveal about, just made it part of conversations.