会说话了就可以了，而且一定要先从听力抓起，再是口头表达，一开始不要先多说，先让他听懂，有了一定听力基础和词汇量后再开始说，推荐一个软件Rosetta Stone，美国的一些小学用它来教美国学生英语的，对提高听说能力很有帮助，绝不会有哑巴英语。

The impression I have is as early as possible. You may have to ask an education professor if you want a more researched answer.

yes ,thank u .i am ready to let him watch mickey mouse in english

You can already start now, but I would recommend that you only teach very basic expressions and the most important thing is to get the soundings correct, and then when he turns 5 it gets easier to learn and also he can learn faster.

Best time is now and the earlier, the better!