My son is 6 years old. He loves astronomy so much. What is the most suitable telescopes for him? Is it 30mm, 50mm, orbigger one?

While my kids were still at home, their grandfather gave us a 15/45 zoom X 40mm telescope. Nobody ever used it much. Its field of view was so small, that it was very difficult to find anything. I am not sure you want to buy a 6 year old an expensive telescope. Perhaps select a larger, inexpensive telescope that compromises on the power. The 6 year old will experience the magic of seeing things somewhat larger. It will gather dust on the shelf if he can't get it focused on what he wants to see.

I have to agree with too much power to be a problem. The more powerful the telescope, the faster the stars apear to move.
If you have a drive too keep up with movement, then the more powerful the better.
You can go to Google Earth, switch to sky view, then type in "Saturn" or "Orions Nebula"
To see their position in the stars.