We used the plug in and collar when our rescue dog was suffering from seperation anxiety. It made a big difference and when the plug in ran out without me realising there was a difference within a day or so.
You can get the spray too which you could spray onto a towel for him to sit on while being groomed too for a double whammy effect.
The adaptil collar alone won't make a huge difference it should be combined with behavioural modification to really help. Making grooming sessions short interrupted by fun and treats combined with the collar will help.
Personally, I found it of no help at all with my girl. A friend of mine also worked at a big rescue when they trialled them on new dogs coming in and they found it made no difference to how they settled so didn't carry on with them.
Thanks everyone, when I'm brushing him at home I tend to treat him at the same time like in poachedegg's clip but I'd like him to just let me do it without eventually! Slowly does it I suppose. I think I'll get one and give it a go.