Is there any difference between drop down box and combo box?

I guess he is getting at the fact

• a combo box is a combination of an input text field and a list of options. You can type in stuff in the input field and the list should update to highlight a possible match
• a dropdown is just a list of options that drops down when you click on the arrow button. You can only choose from a list of predefined options

The combobox allows the user to pick a value from a predefined set or input a custom one. The dropdownlist only allows picking one of the predefined values.

Technically, there are no combo boxes in HTML, because the <SELECT> can't be typed into. See the Wikipedia article on Combo Box (emphasis mine):

A combo box is a commonly-used graphical user interface widget. It is a combination of a drop-down list or list box and a single-line textbox, allowing the user to either type a value directly into the control or choose from the list of existing options.

While the definition of drop-down list matches exactly what the HTML <SELECT> element does:

When activated, it displays (drops down) a list of values, from which the user may select one.

So your senior is right.

A combo box should have more than one item on a predefined list always visible to the user and should either:

enable users to select a single item from the predefined list and/or type their own value;

or

enable users to select multiple items from a displayed predefined list and/or add their own value

A drop-down/drop-list box should only display and enable users to select one item from a predefined list.