A boy can remain fertile for 4-5 weeks after a neuter, because sperm that was produced before the neuter can survive that long in the vas deferens.
It’s not likely though that a 6 week Flemish girl would be able to become pregnant. For does sexual maturity occurs between 3 but more like 5 months of age – and it’s true that bigger breeds mature later.
It’s still recommended to wait until 4 weeks have passed after a neuter before you start bonding, because you want the boy to have calmed down hormonal-wise before you introduce the two buns. The first time after a neuter, a bunny can become extra hormonal behavior wise due to fluctuating hormone-levels in the blood.
If you bond them now when the girl is little, you’ll have to separate them again for her spay, then rebond after a month. It’s always best to have a girl bunny spayed, because of the high risk of diseases of the reproductive organs. Apart from that, an intact girl can demand certain services from the neutered male that he can’t provide, and that can cause a bond to break.