He has been living in Switzerland . . . . . . . . two years.

The preposition 'for' is used to denote a specific amount of time in which someone or something was performing an action. For example, 'I have been looking for my dog for some time now.' Here, 'for some time now' represents not an exact but particular amount of time. The word 'since', on the other hand, refers to a particular point of time from when the action started/began in the past to the present. It indicates that the action is continuing. For example, 'Raj has been living in London since 2003.'
Here are a few more examples to help you understand how the two prepositions are used.
Bruce has been travelling for the past two weeks.
Neethu will be here for a few hours.
I have known you since you were a kid.
Shanti has been working as a content writer since 2018.