Unity callback functions should work even though intellisense is not showing it.
C# IntelliSense for Unity messages and C# code coloration for Unity messages were added to Visual Studio in version 2.8 (3.0 Preview).
The autocomplete feature should work for the OnCollisionEnter2D
function.
I was not able to get this working until I updated it to 2.8.2 (3.0 Preview 3) directly from the Visual Studio IDE.
From Visual Studio, click on the circled icon below after installing Unity, Visual Studio and Microsoft Visual Studio Tools for Unity.

It will give you option to update Visual Studio and other plugins. Update Microsoft Visual Studio Tools for Unity and other items listed there.
If you don't see options to update plugins,update it directly from here. The IntelliSense should work after this.