What Causes Involuntary Arm Movements?

my 15 yr old son started having involuntary movements of his right arm this has happened 5 times in the last 8 weeks,it happens at no specific time of day and is not accompanied by any other symptoms, he s just getting over pneumonia but the first arm jerk happened 2 weeks prior to this. mri scan was clear but has to go for a sleep deprived eeg,would anything more serious have shown up by now ?