[ecfisa]

Hola Miguel.

No es necesario que el campo sea booleano, para eso están las propiedades ValueChecked y ValueUnChecked del TDBCheckBox:

Código Delphi [-]
procedure TForm1.FormCreate(Sender: TObject);
begin
  with DBCheckBox1 do
  begin
    Caption:= '';
    ValueChecked:= 'S';
    ValueUnchecked:= 'N';
    Visible:= False;
  end;
  IBDataset1.Open;
end;

El resto del código sólo necesita unos cambios menores.

Código Delphi [-]
procedure TForm1.DBGrid1DrawColumnCell(Sender: TObject; const Rect: TRect;
  DataCol: Integer; Column: TColumn; State: TGridDrawState);
const
  IsChecked: array[Boolean] of Integer = (DFCS_BUTTONCHECK, DFCS_BUTTONCHECK+DFCS_CHECKED);
var
  DrawState: Integer;
  DrawRect: TRect;
begin
  if gdSelected in State then
  begin
    if Column.FieldName = DBCheckBox1.DataField then
    begin
      DBGrid1.Canvas.Brush.Style:= bsSolid;
      DBGrid1.Canvas.Rectangle(Rect.Left, Rect.Top, Rect.Right, Rect. Bottom);
      DBCheckBox1.Left    := Rect.Left + DBGrid1.Left + 2;
      DBCheckBox1.Top     := Rect.Top + DBGrid1.top + 2;
      DBCheckBox1.Width   := (Rect.Right - Rect.Left + 4) div 2;
      DBCheckBox1.Height  := Rect.Bottom - Rect.Top;
      DBCheckBox1.Visible := True;
      DBCheckBox1.SetFocus;
    end
  end
  else if Column.FieldName = DBCheckBox1.DataField then
    begin
      DrawRect := Rect;
      InflateRect(DrawRect,-1,-1);
      DrawState := IsChecked[Column.Field.AsString='S'];
      DBGrid1.Canvas.FillRect(Rect);
      DrawFrameControl(DBGrid1.Canvas.Handle,DrawRect,DFC_BUTTON,DrawState);
    end;
end;

procedure TForm1.DBGrid1ColExit(Sender: TObject);
begin
  with DBCheckBox1 do
    Visible:= not (DBGrid1.SelectedField.FieldName = DataField)
end;

procedure TForm1.DBCheckBox1Exit(Sender: TObject);
begin
  IBDataSet1.Edit;
  IBDataSet1.Post;
end;
...

Saludos