Is your boy showing signs of readiness? There's checklists floating around on the net, you can google them. It makes it a lot easier if they are ready.
I tried when my son was 18 months with no luck. By 21/22 months he was understanding the concept more and we started toilet training him again and it was a lot easier by then. He was toilet trained just before his 2nd birthday.
Some toddlers may start earlier, or later, depending on when they feel ready