If someone had colon cancer a year ago how often should he get a colonoscopy and do all colon and rectal cancers starts from a polyp?

It depends, and yes. If the colon cancer was small, repeat colonoscopy should be done every year. If it was big or obstructed the colon, repeat colonoscopy might be necessary in 3 or 6 months. Ask your oncologist. And yes, all colon carcinomas are believed to start as polyps.
Monitoring. Monitoring 4 cancer recurrence is an important part of health maintenance. Monitoring can also early detect many other health issues. Typically your regular doc as a team member with your oncology team discuss the best monitoring for you. Typically at the one year anniversary the date for the first colonoscopy is determined. Take family with you to this important visit, ask questions, be well.