Despite what, I think, some YTers think, not all groomers are bad. How you researched this particular groomer and looked for reviews online? I would ask them how long it is going to take and if they can call you plenty of time in advance of when he will be ready. Be very specific with what you would like. Don't assume that they will or won't do anything. I think it helps a great deal if you have a picture.
Look around when you drop him off and make everything is acceptable for you. If you aren't comfortable, leave.
Good luck! I'm sure he'll do fine.
ETA: Also, if the groomer is far from your home, it'd try to stay close by (maybe you could go shopping or something) in case you are needed.