How to explain to manager that the staff is most important resource of hotel and he needs to care about it?

Staff is a unique resource in a hotel that cannot be imitated by competitors. When a hotel manager understands this, then he/she will equip the staff will knowledge, skills and instill the right attitude towards job. The manager will have to keep training these unique resources, motivate them and they will always deliver the expectations of the hotel. It means the hotel will always be on the lead of competition.

It's very difficult to explain anything to any manager, and I suggest you don't try it.  Instead, focus on allowing him to understand for himself.   Something as crude as leaving out an appropriate article for him to find might be helpful.  Another possibility is to have a meeting about guest return rates, and talk around the subject until somebody mentions customer service.

I most cordially disagree that staff is the most important asset of the hotel, although it is certainly very important.  For a hotel, the location is the most important thing.  If nobody wants to go to where it is, it doesn't matter how great the staff are.