Um hm I agree with the others, especially the encopresis.
Does your son have a history of constipation and limited diet?
If he's hiding his soiled underwear, he's embarrassed, and he cares. He's not doing this to act out, and he's also not a child who doesn't care about soiling.
He cares, and it's accidental.
In my experience, adding fiber to his morning - bran or citrucel - will cure this.
Has there been any new happening in his life, new sibling, change of school, does the teacher make sure he gets potty breaks ,is it perhaps staining not a full poop sometimes children do not wipe too well. Be patient with him ,ask him if he is upset about anything, very often nothing physically is wrong they are upset by something ..good luck
It is possible that he has a GI track disorder. If you were you I would have him checked for Chrons, Colitis, IBS, or Ciliac Disease. All of these can have an impact on a child's bowels. He is hiding it, because he is ashamed of not being able to hold it in.
Initially, just put him into big-boy pullups without comment. He's hiding the pants because he's embarrassed or afraid of getting into trouble. See his doctor and ask about encopresis.