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Try to do this in person instead of on the phone. Wait for him to have a few moments alone, or text him saying you need to talk to him about something and plan a time and place to meet. Having the conversation privately, especially when his girlfriend isn’t around, will help you talk about your feelings without feeling pressure or extra nervousness.  Avoid having this conversation over text, as you don’t want his girlfriend or anyone else to find the texts and read them. You might text him, “Do you have time to grab a quick coffee this afternoon?” or go up to him when he’s alone and say, “Can I talk to you for a minute?” Instead of immediately admitting your feelings, ask him about his day or talk about something you both have in common to start a more natural conversation. This will make you less nervous and help the conversation flow more naturally.  You might say, "Thanks for meeting with me, I know you just had a kickball game. How did it go?" You don't need to chat about things for too long, but asking a question about his day or telling him something about yours is a good way to help you both feel more comfortable. This is your time to tell him how to feel, to get your feelings off of your chest. Be honest about how you like him, but realize that he’s still in a relationship and might not give you a response. After you admit your feelings, you’ll likely feel much more free to move on.  You might say, “I know you have a girlfriend, but I needed you to know that I really like you. It’s okay if you don’t respond, I just wanted to get it off my chest.” If you think you'll be able to maintain a platonic friendship even after telling him, you might say, "Even though I told you how I feel, I'd really like it if we could still be friends." Whether he says he likes you or he doesn’t, it’s best to set up boundaries so you don’t interfere with his relationship anymore. If he does like you, make it clear to him that he needs to respect you and his girlfriend by being honest and making a choice. If he doesn’t like you, distance yourself from him and avoid any flirty behavior to respect his relationship and help you move on.  If he doesn’t respond at all, take this as a sign that he doesn’t like you back and try to move on. If he does like you, now the ball is in his court and it’s his turn to make a move. For example, you might say, "Thanks for telling me how you feel. I think it would be best if we didn't talk for a while to put some space between us."

Summary:
Find a time to talk to him when he’s not around others. Ease into the moment by starting with a simple conversation. Tell him your feelings honestly without expecting him to answer. Set boundaries for the two of you depending on how he responds.