Separation from loved ones is very painful for kids. I've so often heard, "Kids are so resilient."
Yes, some are. But even the stronger ones suffer greatly when a loved one is lost.
We adopted my son from Korea when he was 18 months old. He lost his biological mother, culture, home, language, and any attachment to older girls at the orphanage who cared for him.
When he came, he was dazed after the long plane ride and still weak from a bout with the measles.
Thanks to a total flip-flop in day/night time in US vs Korea, he couldn't sleep at all the first night. Didn't sleep well for three nights. Didn't nap for almost a week. Often, he cried loudly and non-stop. He had night terrors. He wouldn't let me out of his sight. But he seemed to recover and at the end of four weeks, seemed "fine."
Yet all through his life so far, he has had trouble with separation. He reacts with anger at any significant loss. I saw the same phenomenon in children of all different ages whom I placed in foster or adoptive homes.
Don't assume the child is "fine" because he begins to eat, react, and socialize normally. There is often a volcano under the smiles. Depression under the fatigue and underachievement.
If the child has had a significant loss (and certainly shelter kids have had many), read stories to him that deal with moving, death of a grandparent, parent, or pet. Give the child toys that will express anger or allow destruction--soldiers, a doll house family, a doctor kit (you will get lots of shots and reflex testing), legos, blocks, and so on. Don't be afraid of his anger. It will subside. And the volcano will get smaller.
With that process, will come tears, frustration, questions, and memories.
You don't have to be a therapist to provide significant emotional help to a child who needs it. You're the one who knows him best and the one who will live with the volcano if it isn't recognized.
Give it a shot. I bet you'll be glad you did.