Looking for counter-arguments concerning the melting of glaciers in the alps in the context of global warming
He concludes that it must have been much warmer in times when mankind was not yet able to bring CO2 in the atmosphere and there is no causal relationship between the melting of the glaciers in the alps and the amount of CO2 in the atmosphere.

This incorrectly collapses several things.


Mankind is not the only cause of CO2 in the atmosphere. There have been warmer periods caused by CO2, but it was not man-made.
I think you slightly misrepresented his words by quoting no causal relationship between the melting of the glaciers in the alps and the amount of CO2 in the atmosphere, but actually meaning is no causal relationship between the melting of the glaciers in the alps and the amount of man-made CO2 in the atmosphere. If not, that would mean he is denying any relationship between CO2 and temperature, which would disqualify him as a knowledgable person.