My 9 month old baby has very low muscle tone in his back he still dose not sit up it's not a neurology problem ped has sent him to physio im worried?

Confusing question: A kid od 9 mo should sit well and i don't understand why you say it is not a neurologic problem.The nervous system controls the posture, so it is involved.A neurologist may not have much to say at this point, since the issue may resolve or persist.It may take time before enough clues are available to put a label on the issue. Work with your team and be patient. It may or may not be a problem later.

Motor milestones: It seems like his milestones are delayed certainly related to poor muscle tone. The main question is if there are other delays as well ? Would consilt with a pediatric neurologist as well and recommend testing for some rare inborn errors of metabolism.