My sons poop looks like tiny hard pellets. should i be concerned?

Constipated...: Most likely he is suffering from constipation. Trying to increase his fluids (prune or apple juice works well), and decrease milk intake if he drinks a lot. Depends on if this is a long standing problem or if he has pain with defecation which is causing him to withhold, treatment with a stool softener like MiraLAX (generic = polyethylene glycol) may be needed, but i would start this with his doc.