To give a compliment to your boss or supervisor, you can say:
"It's a pleasure to work with you." (or It's a pleasure to work for you, if he owns the business)
"I appreciate having you as my boss."
"You are an excellent supervisor."
"Thank you for your encouragement and help."
"I appreciate the way you run the business."
"It's an honour to work for you."
"Thank you for training me and guiding me in my work, I really appreciate it."
"Thanks for giving me work that is challenging and helps me grow in my field. You are a good team leader."
"Thanks for your care for us, your employees. We are glad to have you as our boss."
"I learn a lot from you, you are a great team leader."
"I appreciate your leadership skills."
And so on. You can think of some of the things you sincerely appreciate about your boss, and just say so in a simple and sincere way.