Check his skin, look for hot spots or anything like that. Where was he groomed? Is it somewhere he's been before? How do they bath the dogs?
Reason I ask is because some places do a 'quick' wash on the dogs, they don't actually get them lathered up down to the skin and rinsed again. I went to a grooming place to bath my guys once (they had the self serve thing) and they use a machine that recycles the shampoo water - it goes onto the dog, down into the drain and back out the tap. So by the end of the day or whenever they change it, it's pretty dirty water with not a lot of shampoo 'oomph' to it (I grabbed a bottle of shampoo off the shelf and used the 'rinse' water to bath the dogs). Another place had the shampoo so dilute I couldn't get much lather, also self serve but they said I was the first person to ever complain. It is much faster to put a weak solution of shampoo on and quickly rinse it off, dry the dog, trim the dog and then spray perfume on so they smell nice, so some places do that.
If it's his regular smell, I would suggest switching foods. I feed raw and the dogs don't have much of a smell to them unless they're kind enough to roll in something for me
, but I know some kibbles don't work well with the dog's chemistry.
Kinda like my other half and seafood.
The other thing that can happen is shampoo gets left on the skin, or in the ears, and a hot spot or infection starts, and they can smell.