My oldest son was climbing the walls for food at 3 months old, and I know lots of older moms who said they started their babies on rice at like 2 weeks. I think the thought on when to begin real food changes as often as everything else in raising babies. That being said, my youngest was perfectly content with only breastmilk at 6 months, and I only started food to make sure he wouldn't reject it later. I think it just varies from child to child. Also, when I asked my doctor about the 6 month rule, he said it's a lot of really good theory not backed up by much research.
Anyway, if he's eating solids and formula then he should be having big boy poop. It depends on what you mean by hard though - if it's dry or he's straining too much or in pain then call the doctor and ask the best thing to do for constipation. It could be a passing phase or like others have said you may have to change formulas.