Recommendations for local dog trainers who specialize in older dogs/aggression issues?

I have an 11 y/o scotty-corgi mix (allegedly that's what he is, but no one is really sure). I've had him for 10 years. He is incredibly high-strung, anxious and unpredictable. He never really relaxes. He is quite good with some things, but with others he's rather high-stress. He unfortunately has a history of snapping at strangers when they approach. Because of this I do my best to maintain a wide distance from other people/dogs when we are on walks, but when you live in a city sometimes people just sneak up on you. I've tried really hard to work with him and research things online but nothing seems to help too much. I've also tried fluoxetine with him per his vet's recommendation but never noticed much of a difference. I'm hoping to move to another country within the next couple years and take him with me and would really like to work on some of these issues before doing so. He is a rather mature dog at this point but he's really smart and I have observed him learning things from other dogs and even the cats.
Any recommendations for local trainers in Kzoo or surrounding areas would be greatly appreciated. Thanks for reading!