8 yo child with rt sided back pain x few weeks,
difficulty urinating, painful to bow head. Renal US negative

What has your paediatrician diagnosed?

You should really get him to a doctor. This is beyond the scope of this board.

Not much of a description.
Male?
Where exactly back pain?
Possible injury?
Swollen lymph nodes neck, back of head, under jaw?
Dehydrated? Bad tooth? Appendix? Inguinal hernia? Yes, children get hernias.

Your doctor can SEE. We can't, and you have given us much too little.