I wonder what the brace height is on his bow? On some kids bows, the brace height is pretty low, and the feathers almost reach the bow shelf when the arrow is nocked on the bow. If that is the case, you might consider a higher brace height, if possible within bow specs, or shorter feathers.
Where is the nock point? For someone who is learning to shoot the bow, it is probably best to set the nock point at some standard position, like 1/2 - 5/8" for split, or 3/4" for 3 under. Don't be afraid to raise the nock point up higher than normal if that solves the problem, as a high nock point will be easily corrected by the fletching.
Some bow grips are built such that the bow hand is very close to the arrow. I have bows like that where the feather brushes my forefinger with every shot. Some bows are built with no arrow rest, and of course the feather will brush the bow hand if the bow hand is being used as an arrow rest.
If he is shooting split fingers, and is pressing down on the arrow at full draw with his forefinger, the arrow will porpouse. An indication of this is if his string elbow is much higher than the arrow nock.
Sometimes it is hard to get an arrow that has a weak enough spine for a low poundage kids bow. Arrow spine problems usually result in fishtailing, but a large enough mismatch could result in porpousing.