Is there any pain when u try to move his right hand? Have u tried handing him a toy and he refused to move his right hand to obtain it? Does he cry if u press at his shoulder or elbow joint?
Sudden cessation of movement usually happens with dislocation which can be easily relieved by an orthopedic surgeon or pediatrician. You should take a second opinion
Next Steps
If the tone of his right hand is lesser than usual, like if u lift it and it suddenly drops, then u have to see a pediatric neurologist