I am concerned that your son is suffering from GERD
. Gastric juice, bile juice can give a yellow tint to vomiting. One or two episodes of yellow colored vomiting is not a cause of concern but if yellow to green colored vomitus persists, it may suggest intestinal obstruction
and should be evaluated without ignoring.
Regarding GERD, as long as your child is gaining weight adequately regurgitation is not a big issue. But if he doesn't gain weight he needs to be evaluated further.
So just keep a watch for persistence of yellow vomitus and weight gain. Take care.