I think the best way to get them comfortable at the groomer, is to first make them completely comfortable with being brushed at home, by you. If you are not brushing every day, starting that would be a good thing, this will get him used to it, and keep any matts/tangles at bay. The tangles on the tummy are probably the most sensitive area. I always keep the tummys trimmed short. Cowboy magic is a good detangler for pupppy coat. Also, if the hair is somewhat thin on his tummy, you can use corn starch on dry coat and just work the tangles out with your fingertips.
The wooden pin brush from Chris Christianson is good on small tangles and much more tolerable for the pups
