I also think it's his anal glands, which would also explaing some of the itching he's beend doing back there. Granted it may not be the only problem causing the itching but it very well may be at least part of the problem.
And yes, not all groomers do it, most of them you need to specify that you want it done. And some dogs may need them expressed more than others, depending on the dog they may need to be expressed once a week or once a year or never. If the anal glands are causing a problem over & over again, diet may help.