No, the correct way to say it would be "He is on the staff in the restaurant." Staff can be pluralized, but then you're talking about the staff from more than one work place. So it's "The staffs of three hotels participated in the strike." It's common just to use "staff" as the plural, though.

Staff is an uncountable noun, therefore it does not take an s in the plural. You would say "he is one of the staff members/ He is on the staff/He is a member of staff

"Staff" means "they", just like the police. For a single person, use "staff member". "He is one of the staff in the restaurant" and "He is one of the staff members in the restaurant" are both correct. If you say "staffs", then I think of wooden poles. "He is one of the staffs..." sounds very unnatural.

Sunny, there is no such word as 'staffs' when you are talking about employees. This is because - in meaning if not in grammar - the word 'staff' already IS 'plural' in a sense, because it refers to the set of PEOPLE who are employed in a company or department. In fact, the word staff - often as not - takes a plural form of the verb eg The staff have gone on strike. The staff aren't allowed to take holiday in August. This clearly shows that you are talking about the whole set of people. If you want to single one person out, you would say 'a member of staff'. NB there are other nouns in English which behave like this, such as committee, council, government, police or team. They are grammatically singular, but refer to a group of people, and so they can take a singular or plural verb, depending on whether you are focusing on them as a single entity or a group of individuals. Try thinking about the word 'staff' as being similar to the word 'team'. You wouldn't say that a person is 'one of the teams' , would you? You'd say that he or she is a 'one of the team members' or 'one of the members of the team'. And it's the same with 'staff'. 'Staff' means the whole team, ie all the employees, while 'a member of staff' or 'staff member' means just one of them. I hope that's clear.