How can I get someone to become a Christian?

Before I answer your question, I have a few clarifying ones of my own. First, Ты dated someone Ты don't consider a close friend? Maybe the breakup made Ты feel less friendly towards him, but Ты must have been Друзья at one time.

Also, how do Ты know he's depressed? And if he is depressed, Ты don't know if the cause is spiritual или not. If Ты want to help him, be there for him, in a nonjudgmental way. Listen to him еще than Ты talk. "Be quick to listen, slow to speak and slow to anger." - James 1:19

As for your Вопрос - Ты cannot make anyone become a Christian, nor should you. In nine out of ten cases, proselytizing will only push people further away from you. All Ты can do is explain why you have faith, from your own perspective. Explain why God and Christ matters so much to you. Share your story, but don't force your choices on him.

He may have seriously considered Христианство when Ты dated. He really may have, and decided that it just genuinely was not the spiritual choice for him. That's his decision to make, not yours. If he was lying, and saying he was Christian just to дата you, well, that sucks, but there's nothing Ты can do about it. Except, maybe, ask him if that's what happened.

But it sounds to me that by "not giving up" on trying to make him a Christian Ты might have pushed him away. I think your focusing your attention on the wrong goal. Instead of trying to force him into Christianity, treat him like a human being, with his own beliefs and worldviews, even if they are different from yours. Forgive him his flaws, his lying, for example, and Любовь him nevertheless, for who he is. Not who Ты want him to be.

If Ты spend your energies trying to turn a person into something he's not, trying to force someone to see the world Ты do, Ты will not only push him away, you'll burn out. Some people just don't want to be Christians. That's their choice, and Ты should respect that. Similarly, they should respect your choice to be Christian. And if he doesn't, if he treats Ты badly, then stop being his friend. Ты can forgive him for that, but Ты don't have to do that at the cost of your own self-esteem.

As it has been said, Ты cannot make someone a Christian. That is thier choice. The bible tells us that not all will believe.
Remember, we plant the seed, but God makes it grow. Ты can continue to pray for him and ask that the Holy Spirit works in his heart.
Ты don't have to pressure him. Accept him as he is. There's an old saying, let go and let God. There are things in life that we cannot do ourselves. That's when we take our hands off and give the situation to God. Ты can trust him.

I'm going to tell Ты a personal story.  My friend says she's Christian, but the way she acts tells me she's not. I worry for her, soon I put my faith in God and just prayed for her constantly. One день she texted me saying she was confused (boy problems). And asked if I could help her I сказал(-а) yes. I listened to her. In the end she said you're such a good person and I think it's really good and cool how you're super involved with god and I want to start becoming closer to god.
Me- Ты are a smart and sweet girl. I know Ты can figure any thing out if Ты set your mind to it. Ты just need to work on have a good relationship with Him and Ты wont feel so stressed out over things. I'm not saying Ты are. But.  You are an amazing friend with the true talent of being your self. It's hard to find your self in this world. Trust me we just did a bible study on it.
Her- yeah I'm trying to find who I really am. in 6 grade, I chose the wrong Друзья who are no longer my Друзья and I had to try so hard to please them and. I wanted to be like them and fit in. but I want to find the real person in side of me and start being that person.
Me- Trust yourself and your friends. To start a good relationship with God Ты first need to confess of your sins to him. And prove that Ты are sorry Ты did them. Then put ALL of your trust in God. "put all Ты worries on Him for he cares for YOU."
Her-you know what.. I'm gonna do that. I'm gonna pray and ask him to forgive me for my sins. and I'm going to trust that tomorrow he let's what he thinks.  is right to happen. even if it's not what i want.
Me-And think about what and who Ты want to impress of it's not God, it needs to be it hard but I'm here for Ты and so is God.
Her-  thank you! that really helped me alot and now I realize I need to pray to god more. and I feel different. you're the best! 
Me-You on the right track (her) Let God guide you. Your ready to accepted Him now and he willing to let Ты come to him.
Her-thank Ты so much! I Любовь Ты Ты are such an amazing friend and I know you're always there for me. and I'll always be here for you. 
Me-Yes but Ты turned to God. What did Ты just read?  Her-Jeremiah29:11 "for I know the plans I have for you" declares the lord "plans to prosper Ты and not to harm you, plans to give Ты hope and a future"
Me-Just remember Ты are NEVER alone. He is always with Ты He never left you.
Her-yes! haha I already feel so different in a good way after talking to Ты about all of this

Give him a Bible. Ты can find some verses and read them to him. Also Ты could take him to church with you.

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the awnser is, Ты cant. its all there own free will. we cant force it on them. they have to want to make the desctition too there wills and there lives over to the care of god themselves.