Be specific about what you like
Compliment his appearance
Compliment his style
Compliment his confidence
Compliment his intelligence
Compliment his sense of humor
Compliment his effort
Compliment his skills
Compliment his kindness
Compliment his personality
Compliment his achievements
Compliment his taste
Compliment his voice
Compliment his presence
Compliment his reliability
Compliment his work ethic
Compliment his leadership
Compliment his creativity
Compliment his smile
Compliment his energy
Keep it genuine
Keep it simple
Say it directly
Match the compliment to the moment
Avoid overdoing it
Make eye contact when you say it