How to give opinion to my professor about writing LoR?
I have two suggestions. The first is to give him an outline of what you think your accomplishments are and say something about your enthusiasm and future goals. Or, at least, offer to give him such an outline if it would help. This is just to give them a few ideas. 

The second, probably more important, is that you ask another, more experienced, professor that is also writing for you, to speak with the advisor and also give them ideas about how to frame such a letter. It is normal for one professor to help out another, newer colleague, on their responsibilities. 

I'll note that some letter writers actually ask the candidate for a first draft of the letter they will send. There is some controversy about this, but it is common enough that you shouldn't be surprised if it happens.