Mystery

My son has had a normal MRI of the brain.  His aldolase levels are high but his CPK levels are normal.  He has always had low muscle tone especially in the oral area and legs and trunk.  He used to have it in arms too but thanks to PT he is now extremely strong there and can pull himself up on many things with his arms!  He is a bright boy but this hypotonia (low tone) is a driving mystery.  His neurologist right now is repeating the blood tests to double check results for any lab errors but any clue as to what could cause this?  He is developmentally delayed in speech, ot, and Pt.  I would like to know what is causing this so I know what to expect and how to help him best.  Any clues or suggestions out there?

Thanks!