How can I use this sentence with purpose?

"I'm waiting for him to come in order to have dinner together."  This is very formal. "In order" is what makes it formal. I wouldn't use this one. 

"I'm waiting for him to come so that we can have dinner together." This sentence is good! However, if you are talking about a friend who you want to have dinner with, you can say it like this:

"My friend and I are going to have dinner together. Now, I'm waiting for him to come so we can eat."

There are many more ways to write this sentence. 

How can I use this sentence with purpose?

The sentence I was wondering about was this.

"I'm waiting for him to come."

and how can I add purpose('to have dinner together' for example) of the reason why I'm waiting?

Should I say like

"I'm waiting for him to come in order to have dinner together." This sounds too legalistic/cold/logical.

or

"I'm waiting for him to come so that we have dinner together." This is okay.

or else Is there's the other another way to express this situation?

"I'm waiting for him to come to have dinner together" would be the best.

Many thanks in advance!