How do I convince my boyfriend to be honest about his feelings?

Okay, so I have been in a relationship for about 3 years (in July). My boyfriend and I used to talk about marriage and children constantly, and recently he has been avoiding the discussion. Last night, he finally confided in me that he doesn't want to get married, and isn't sure if he ever will. He says it isn't that he doesn't want to marry me, just that he doesn't want to get married in general. He admitted that he only talked about it in the past because he thought that it was what I wanted to do. I have had problems with him about this in the past, where he'll say one thing, but only because he thinks it's what I want to hear. How do I convince him to be honest about the way he feels? I feel like he's afraid I will get mad or upset at him, but it upsets me more when he lies. There can't be a relationship without honesty, so how do I fix it? I want to stay with him forever, and would like to eventually get married. I don't want to force him into it, but I do hope he will eventually decide he wants it. Honestly, I would be fine with just being "in a relationship" with him. But, I need him to be honest with me.