in order to answer your question few things needed first 
was your child able to hold his head at the age of 3 months ? 
does your child choke a lot? 
does he have a recurrent respiratory infections ? 
does he talk , smile , laugh , able to play ? 
did he was able to sit and walk and lose these abilities? 
if the problem is only motor ; he can't sit or move properly, then maybe the physical exam shows 
hypotonia  ( no tone or decreased tone ) which some times come as an islated disorder , or maybe accompanied by other findings which need to be investigated thoroughly, because many disorders can cause that . 
but 
physiotherapy can be helpful , schedule with an expert physiotherapist and learn how to do it and do it on regular basis 3-6 times a day.  start with the back muscles , because if your child couldn't sit , he will not walk , he needs a good back muscles .