Nothing you can do apart from keep talking to him. Some birds will never talk, some will talk only when alone, some will talk but wont be understood because their ''voices'' are not clear so it sounds like a load of random noises or whistles.
First of all do you need him to talk to make you happy? From what I have learned from mine is that they soak up info and only use it when needed. Sometimes mine say phrases very clear that I have never heard from them before. He may just be storing it up. Don't answer him when he whistles for you.
Same advice. If you want him to talk, just keep on talking to him all the time. Dont whistle yourself, but sing instead. Mine does store it for a long while sometimes, and sometimes it comes out right away.....