I don't remember how I did it...I was using full sentences at 11 months. However, as a teacher, in order to get students to use more than one-word or one sentence answers, we use open-ended questions. That's a question that can't be answered with just "yes" or "no." For example, "Did you have fun in school today?" is a closed question, it only requires "yes" or "no." "What did you do in school today?" is an open-ended question. Try to ask your son questions that require longer answers or explanation, and probe his answers with related, open-ended questions. I can't promise this will work, but hopefully encouraging him to speak more will allow him to feel more comfortable giving longer answers.