I got curious about this but it seems that this is also the case for all his siblings. His bio dad gets it (EUR 300k/y) because of a transitional compromise but the future princes aside from the presumptive heir to the throne will not get it.

Not that they might be any close to being destitute given the estate size.

As for him, I suppose you can sue for backpay child support.

he can now borrow against that inheritance