Modules & VBA :: Disable Button When Text / Combo Fields Are Null

On a form, I want to disable the save button, 'cmdSave' whilst the form's mandatory fields have been left blank.

I've put in a smart tag, called, 'Req' against each required field and have used the following code on the forms current event.

Code:
Private Sub Form_Current()
Dim ctl As Control
For Each ctl In Me.Controls
If ctl.ControlType = acTextBox Or ctl.ControlType = acComboBox Or ctl.ControlType = acListBox Then

[Code] ....

The save button is disabled, but it won't enable again after each field has data entered against it.

I also have this code in the AfterUpdate event in each required field:

Code:
Private Sub cboErrorID_AfterUpdate()
Call Form_Current
End Sub