You don't have to worry too much, just continue to be vigilant and think prevention.
In the medical world choking is called an airway obstruction injury. Children, especially those under age 3, are particularly vulnerable to airway obstruction death and injury due to their small upper airways, their relative inexperience with chewing and their natural tendency to put objects in their mouths.
From what I could find out on the Safe Kids Worldwide website, In 2001, there are about 60,000,000 children <15 yrs old living in the United States and only 169 children died from choking (30 percent food and 70 percent nonfood) and more than 17,500 children were treated in hospital emergency departments for choking-related episodes. Children are at risk from choking on small, round foods such as hot dogs, candies, nuts, grapes, carrots and popcorn.
So I think you did a good job in supervising your DS while he was eating so continue doing that…but you might want to stay away from chunks of banana, hot dogs, and other round foods. And also reviewing first aid and CPR instructions like the other poster suggested is a good idea too.
JGG