If he did a good job, then don't have any regrets about tipping him.
He'll remember that you tipped him. And next time you bring a critter in to get stuffed, he may bump you up to the top of the list, just to ensure that you keep coming back. Taxidermists LOVE repeat business.
You could bring him 5 squirrels a year, and he'll like your business more than the guy who brings him 1 deer every 5 years.
For a while, I was bringing my stuffer an average of 1 critter a month. Hogs, coons, ducks, deer, coyotes...whatever me and my buddies shot. The more I brought in, the more of a discount I got and the faster I got em back. I tipped him $20-$50 each time, depending on the job.
I think you did good tipping him, and I'm sure he appreciates it! Everybody loves a gratuity.