the couple horses I have had that were food aggressive (not so much aggressive, more pushy, snatching hay and the like- the mule would threaten a kick), I would go into the stall or pen equipped with a lung whip. They want to be pushy, they got a good wack (the kick threatener got as many wacks as I could get in in 2 seconds) and I made them leave the stall or go to the other side of the pen away from the food. I made sure that they knew that being pushy, snatching hay, and threatening kicks just made that meal more delayed, and that I was boss and I decided when they could eat. Never took more than three times, and my horses aren't scared of the lunge whip because I also make a point to pet them with it...but they are aware that it can bite.
As for your son, I think it depends on his age and horse aptitude (Ive seen five years olds who have alot of horse sense- but they are far and few inbween). Unless he can go in there and give it to em effectively without coming off as a bully, or backing down, I would keep him out of there at feed time.