We use the present continuous a lot for future events. Actually, imagine the event is entered in your diary or calendar as an appointment. In such case you use the present continuous (be + ~ing).

This is the "going to" form. It is not the same as the "will" form. He has already decided to tie the knot next month. Thus you must use the "going to" form.

Because English doesn't have neat little verb conjugations for future actions, we have to "construct" our future tense. :) As well as using "going to..." and "will...", if you add a future time to "to be (verb)+ing", then you have a decided future event. It's that simple. As for "will", it only counts as a sure prediction. Fairly close in meaning, but the "to be (verb)+ing" with a future time has a more definite feel to it because you're using a present tense action instead of a modal verb. If you're not sure that it is next month that he ties the knot, then rewrite the sentence, eg. "He may tie the knot..."

i found I have answered a wrong answer.. sorry abou that. xD

你应该好好研究一下tie the knot ，这里的tie 已经失去了打结的含义，根本没有action verb 的含义，变成了结婚，是一种slung ，这种东西直接影响了你，在英语中stative verb 在使用时，ing 的形式已经失去了动作的含义，我们在说某人已经安排去做什么事情，这时可以使用i'm+doing 。这里的doing一定是stative verbs，不是action verb 了。如果是action verb 的话，那么就成了present continuous(现在进行时）了。