Do you mean cystoscopy (scope), not cystostomy (stoma/hole in the belly)?
I'm still surprised that the doc recommended circ. And not in a good way surprised.
I hope he recovers quickly. The valve ablation is painful enough.
You can refuse the LP. It is your child. If there is no suspicion of meningitis, the test is not ONLY painful, but it is very dangerous. Especially in a child with any abnormalities of the spinal area (deep sacral dimple), an LP should ONLY be done when absolutely necessary.
I'm praying they have chosen the right antibiotics and that the sepsis is quickly under control.