Shall I submit an original invitation letter stamped by the Chinese ministry of foreign affairs?

The invitation letter should better be original. Besides, you need to get a covering letter from the Economic & Commercial Office of Chinese embasy.

Thanks Mr. Duke.
I have another question, the Chinese company that sent me the invitation letter requested: "We need his professional knowledge in the field of steel structure on technology and business. We'd appreciate if you could issue 2 years Double entry visas to him."

Knowing that I will be a representative for my company until the project is finished by the Chinese company

Is it possible to provide two years visa?
What catefory will be, F or Z?
Is it possible to accompany my wife and son later?

Thanks in advance.

Normally, you will only be issued with a F visa, an dyour wife and son have to apply for a L visa to accompany you to China. They will be only allowed to stay in China at most 30 days each time.

For my wife and son when they stay 30 days, they have to re-enter or it can be renewed from China?

Is there other solution to accompany my wife and son?

Thanks

I think they may have to re-enter fron Hk, Macau or your home country. Sprry, I don't think there is pther soluntions. You may turn to the local China embasy or consualte for practical help.