I would ask the previous groomers what they used. They just might tell you.

 

Have you talked to your vet about maybe a mild tranquilizer? You're probably going to have some problems using the clippers on him yourself. It took me a couple of months of working at it to get my young dog where he'd let me use a Dremel on his nails, and I think getting one to tolerate clippers without a fuss would be a similar project.

 

Have a look online for Chirag Patel’s Bucket Game - it’s on YouTube, maybe under his name or maybe his company’s name, Domesticated Manners. It’s about consensual handling, so it teaches your dog he can say ‘stop now please’. And, you listen and stop for a break then go back to the grooming later. It helps build your your dog‘s confidence in husbandry tasks. He might look a bit lop-sided for a few days, but he’ll be a lot less stressed.