thanku for your query,
Well i understand your concern. Normally a baby starts walking with support 10 to 12mths and without support around 12 to 15mths,stands with support around 10mths and without support should be by 13mths.Now as u said he is 18mths, is not able to stand independently , and has shaky limbs
with legs very flexible,low msl tone,flat feet, and left foot is turned out . After reviewing all this it seems you need a orthopedic reference ,who will rule out flat foot
,congenital talipes equinus varus in which foot is turned out. He has low muscle tone
of leg probably because the baby is not using them much and flexibility of joints has to be assessed properly.
Also how are the other milestone like speech, and social development.
He needs to examined by orthopedician who will tell the basic diagnosis and accordingly managment and physiotherapy
I hope i have answered your query,