Eight ways.

1. Specific compliment: "You were so patient with your sister yesterday."
2. Physical affection in small moments (hand on back, lingering hug)
3. Remember what he's working on and ask follow-up questions
4. Plan a small thing he'd enjoy without being asked
5. Write a handwritten note
6. Tell him you're proud of him, specifically
7. Speak well of him when he's not in the room (he'll hear)
8. Say "I love you" at unexpected times, not only as a greeting