My 2 year old son has a hard time pooping. when he finally goes its hard and they are tiny balls of poop. should i be concerned?

Yes: Discomfort during stooling @ this age can set up a pattern of holding/retention that can prolong the problem for years.The final segment of the large intestine can balloon & become weakened. Early & long term effort to keep stools soft & attention to regular emptying helps.Therapy for twice the duration of the issue is often necessary as recurrence usually occurs with shorter treatment.

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.