He works in a hospital. (which means is job is working in a hospital)

He is working in a hospital. (which means he is working right now)

His work is in a hospital. (unusual expression)

He worked in the hospital. (which means he worked in the hospital in the past)

He works at a hospital. (at is okay but unusual in this situation)

In hospital is a British expression while in the hospital is an American expression. Both expressions are okay.

如果你要參加高考，在高考之前，請忽略以上內容，並記住華人給它賦予的意義：

In hospital 表示住院

In the hospital 表示在醫院辦事等（除了住院之外的一切事務）