My 6 month old hasn't pooped in 3 days. He tries but nothing comes out. He eats oatmeal/veggies along with formula. What can I do/when to worry?

Constipation? If this is a change from his normal pattern, he may need stool softeners. First, i'd advise trying baby food prunes daily and increasing his water intake. If there are no results from this, he should be checked by his doctor to ensure there are no other underlying causes; if there aren't, his doctor can recommend a gentle laxative at an appropriate dose for his size.