Men enjoy compliments as much as women do, so compliment his appearance recently or his performance on a recent project. Success at work is important to men. Compliment his promotion or new skills.
Show gratitude for things your boyfriend has done for you. Tell him that you know his time is valuable, and thank him for calling. Compliment him on things in which he takes pride. If he likes music, compliment his taste, and ask him to help build a playlist for an upcoming event. If he's a fitness buff, compliment his recent progress and ask him to help design a fitness plan for you.
Men enjoy hearing that people enjoy spending time with them. Tell him you miss him when you're apart. Acknowledge efforts he made to do nice things, such as cooking or buying a gift, even if the results weren't perfect. Don't be tempted to qualify your acknowledgement by adding advice on improvements unless he asks.
Tell your boyfriend what you'd like to do the next time you meet. Men are frustrated by indecision, so they find decisiveness attractive. If he did something the last time you met that you really enjoyed, tell him that. He's more likely to repeat actions that he knows are appreciated.