In your code I see that you are defining the ComboBox in a template explicitly. There is no need to do this. RadGridView provides a specialized column - the GridViewComboBox column. It would create the combobox and set bindings for you. You just need to define such column and set a few properties.
The approach is described in details in this article
In case you have further troubles , just send me your project and I will gladly modify it to fix the issue.
*In order to attach the project files you will need to open a support ticket.
the Telerik team