I usually cook all my DS's meals, except when I...

I usually cook all my DS's meals, except when I run out and I'm not organised, then I will run to the shop and get a can of something. My son is 7mths in a few weeks and I thought I would give him a can of 7-9mth meat and veg. The chunks in it were huge and he gagged on it everytime it went in his mouth. I had to end up giving up for fear of him choking. How do I work him up to chunky food. And if he's almost 7mths why cant he handle it?