Don't forget to get a stool sample to check for parasites. Even non-neglected kids often have parasites (in seattle, over 80% of public playgrounds tested, tested positive for pinworm and roundworm eggs... mostly in the more affluent neighborhoods, since poor children are routinely wormed, and middle/upper class children aren't, which is just kind of ironic, but I digress), and the rate for neglected kids who have worms is sky high. Super easy fix: 2 chewable pills.
Ditto blood samples to test for other imbalances / abnormalities.
Also drug testing if you've had him for less than 1 month. ((If he's been exposed to drugs, know that he'll be going through the same withdrawal phases an adult will... 2 of which include mood swings and excessive hunger)).
Also a full STD panel (most don't actually require sexual contact to contract, btw, although that's the easiest way. Any blood to blood contact, or transfer of puss or skin cells can do it... and again... are commonly transferred to children in unsanitary homes). Several STDs affect the brain, and can cause mood swings/ outbursts/ irregulation of various bodily functions (hunger being one).
There are a LOT of causes for overeating (my own son is on meds that make him perpetually hungry with no "shut off"... while I was in the bathroom a few months ago he ate 6 bagels. Headsmack.), and I'm sure trauma is playing into them... however... DO check for physical causes. Parasites, diabetes, thyroid, drug withdrawal, infection, etc. can all be "fixed" (not all cured, but the excessive hunger can be fixed with treatment if there is a medical cause).
Huzzah! For the psych eval / therapy!! Good on you! (The med stuff isn't detracting from that, just a reminder of an 'easy button' for some of the issues).