der Tisch

an (at) is a preposition that can work with both dative and accusative case.

Accusative is the case of movement towards a direction.

an einen kleinen Tisch

is accusative

He is moving into a sitting position at the table.

Don't worry at this stage too much about the forms. We will tackle that later, but try to grasp the idea of when to use the accusative.

Now he finished the process of sitting down, he is situated at the table. We are no longer talking about the direction of a movement but about the location, where the man is at.

an einem kleinen Tisch

is Dative

When we want to describe a location, we use prepositions followed by a dative.

in einem Café

an einem Tisch

He is sitting down. (direction >accusative) The table is standing in the corner (location>dative)

It would be a different case if we said:

He sits down in the corner.

In this case we would have a movement into the corner. >Accusative

Er setzt sich in die Ecke.