unless he is really small a good fitting 20 with trap loads should not be an issue.
in our Friday night youth trap league I have several lightweights and they all handle standard trap loads from a pump gun well.
first figure out his eye dominace fi you don't already know it , shoot you your eye not to the hand you write with.
fit is huge , up to the cheek into the shoulder , make sure it is in the pocket and not to long or to short for him.
make sure he holds it tight in his shoulder.
posture is next feet shoulder width apart or a touch more stand tall then bend the support hand knee and lean forward so that your weight is more over your front foot.
a demo I do for the kids is to tell them think of a punch it hurts why because it has speed I make a fist and punch my open hand from about 6 inches away you can hear the punch.
then have them stand up strait I place my fist firm against their shoulder the same way I want the gun and I push their shoulder with about the same energy I put in that punch of my hand and they tip back some then have them lean forward anticipating the push and push their shoulder with my fist and they come up to about strait. explaining that because a gun held firm to the shoulder has no time to develop movment they are slowing it down from the very beginning like the push vs the punch.
here is a video to help explain the feet
shotgun stance next have him pattern set up a target stand that is tall about 8 feet set it out 32 yards and shoot some cardboard with a picture of a clay drawn on it on marker so that he can see it well from that distance load the gun for him so he can't see you do it and after about 3 rounds hand him an unloaded gun action closed do not tell him. if he flinches work ball dummy mixing it up till he can not flinch making sure his stance is good and butt pad in his shoulder well.
when he has this down , with an unloaded gun have him follow some clays keep moving and follow all the way to the ground , have him point out where the clay seems to just hang and have him dry fire on the clay. if I can I have them move right up next to the trap house dry fire on a few then live fire on one , then hand him back an empty gun see if he flinches work that ball dummy till he doesn't
remember follow the clay or the pieces all the way to the ground , follow through is important
all this time you are handing him the gun by positioning it right in his shoulder and getting the right fit he needs to make sure his finger stays off the trigger till he is ready and that you maintain safe muzzle direction this is one of the only times we ever had a student a loaded gun and we do it very carefully so that they don't move position and we get the right fit.