My daughter is 20 months also and has just started taking her nappy off consistently and been a 'rudy nudy' as i like to call her. During these nudy times i sit her on the potty every 15-20 mins. This may sound excessive but i figured better be safe than sorry. I have not actually had her use the potty yet but she has used the big toilet twice. (oh and he floor more often, lol) Would he let you know if he needed to go? I dont think my DD knows b4 it happens iykwim as she gets quite upset when it does almost like she is scared at what her body is doing. She carries on with the whole 'uh oh, oh no' thing for ages when she has done her business without a nappy. Good luck starting to TT.