As long as he's being active and burning up the calories boys will eat a lot. My husband friend was and is like that and the joke was "well, he's just filling up the other leg now." He's over 6' tall.
However, the speed is something he should work on. one it's bad manners to wolf down your food. two, you have to give you brain time to process your stomach capacity, meaning you have to get the food there then it sends the signal on. If he's finishing a sandwich is 4 bites, then he's not taking time to chew or he's just taking too big a bite. No one wants to sit with someone (@ school/elsewhere) who looks bad when eating. Our mouths are only so big and putting too much in our mouths truly makes us have to overwork and look un-gentlemanly (or un-ladylike).
Did he start this because he's not having enough time at lunch in school so he's in a hurry. If so, try to determine if it's to school not providing adequate time or him not utilizing that time (Playing instead of eating).
Manners now will gain him more points later in life than just going for quick quantity. He can still eat what he needs, just eat it slower.