My daughter's boyfriend asked her to marry him today. *They have a daughter together* Read details?

They are 16 and they have a 2 month old daughter. Natalie and Branson (my step-daughter and her boyfriend) have been living together for 2 months now. Since Mady was born. He asked us about it a month ago or so, if he could have her hand in marriage and we weren't too sure about it because they are only 16, but than we thought and prayed about it. We figured it would be good for them. They both are going to school. Natalie is doing online schooling and Branson is still in school. They both have a part-time job and they pay for almost 90% of everything for themselves and their daughter. So we thought it would work out and Branson's parents thought it was a great idea, so we agreed he could ask her. When he asked her, she didn't say yes and she started crying. He asked this morning at breakfast. he had made her heart shaped strawberry pancakes (her favourite) and it was so sweet, and than he asked her. All she said was "I can't do this" and than she ran to her room and was crying hysterically. So I went up there to see what was wrong and she said, that she loves him very much but she's not sure hes the right guy for her. I was shocked! I thought she was in love with him, and I thought she would have wanted to marry him in a heartbeat. I know she is only 16, but they both love each other so much, and because they have been living with each other for 2 months now. I would have thought she would have married him in a heart beat. She kept saying, that she knows she wants him in her life and Mady's, but she's not sure if she wants to marry him. Which I think is totally fine and it is her choice, but I am afraid that if she tells him that he won't want to be in her life anymore. It's not that he's that type of guy, where all he wants is and just wants to use hr, but he's the kinda guy that loves her for who she is and cares for her so much that if it wasn't her plan to marry him he just couldn't deal with the fact that they would just be there for Mady. he would be too hurt, I think, and I feel that might make him leave. Because eventhough he loves her very much he is still only 16 and I am not very sure what he's going to do. I just would like some words of wisdom and ideas of what to say to them both. I would greatly appreciate it!!! Thanks!